Solid-phase Extraction 0128169060, 9780128169063

Table of contents :
Series Title
Solid-Phase Extraction
Copyright
Contributors
1 -
Core concepts and milestones in the development of solid-phase extraction
1.1 Introduction
1.2 First generation formats
1.3 Second generation formats
1.4 Sorbent chemistries and properties
1.4.1 Inorganic oxides
1.4.2 Low-specificity sorbents
1.4.3 High-specificity sorbents
1.4.4 Sorption mechanisms
1.5 Theoretical contributions to modeling sample processing conditions for cartridge and disk devices
1.5.1 Breakthrough volumes
1.5.1.1 Experimental determination of breakthrough volumes
1.5.1.2 Estimation methods for breakthrough volumes
1.5.1.3 Experimental determination of retention factors
1.5.1.4 Estimation of retention factors
1.6 Conclusions
References
2 -
Inorganic oxide and chemically bonded sorbents
2.1 Introduction
2.2 Inorganic oxide sorbents
2.2.1 Physicochemical properties
2.2.2 Silica-based chemically bonded sorbents
2.3 Evaluation of formats, sorbent types, and modes of interaction
2.3.1 Sorbent pretreatment
2.3.2 Miniaturization and automation
2.4 Sorbent characterization techniques
2.5 Conclusions
Acknowledgements
References
3 -
Porous polymer sorbents
3.1 Introduction
3.2 Hydrophobic porous polymers
3.2.1 Macroporous polymers
3.2.2 Hypercrosslinked polymers
3.3 Hydrophylic porous polymers
3.3.1 Functionalized polymers with polar moieties
3.3.2 Copolymerization with a hydrophilic monomer
3.4 Mixed-mode ion-exchange polymers
3.5 Conclusions
Acknowledgements
References
4 -
Carbon-based adsorbents
4.1 Introduction
4.2 Carbon-based adsorbent for solid- phase extraction
4.2.1 Fullerenes
4.2.2 Carbon nanotubes
4.2.3 Graphene
4.2.4 Nanohorns, nanodiamonds, nanofibres, and quantum dots
4.3 Conclusions
List of abbreviations
References
5 -
Restricted access media
5.1 Introduction
5.2 Restricted access media
5.2.1 RAM with a physical barrier
5.2.1.1 Alkyl-diol-silica material
5.2.1.2 SCX-RAM synthesized by surface-initiated atom transfer radical polymerization
5.2.1.3 Monolithic phases
5.2.2 RAM with a chemical barrier
5.2.2.1 Mixed-functional material
5.2.2.2 Protein-coated silica
5.2.2.3 BSA-coated carbon nanotubes
5.2.2.4 BSA-coated polymer
5.2.3 Restricted access material combined with molecularly imprinted polymers (RAMIP)
5.2.3.1 RAMIP synthesis using hydrophilic monomers
5.2.3.2 RAMIP synthesis using comonomers
5.2.3.3 RAMIP synthesis with hydrophilic monomer combined with a BSA layer
5.2.3.4 Nonconventional RAMIP material
5.3 RAM sorbents interfaced with LC systems
5.3.1 RAM sorbents in the single column mode
5.3.2 RAM sorbents hyphenated with column-switching and multidimensional systems
5.4 Conclusion and future challenges
Acknowledgements
References
6 -
Aptamer-based and immunosorbents
6.1 Introduction
6.2 Grafting of antibodies and aptamers on a solid support
6.3 Extraction procedure using IS and OS
6.4 Various extraction format and methods
6.4.1 SPE in cartridge or column
6.4.2 Dispersive SPE
6.4.3 SPME, SBSE, and associated methods
6.5 Capacity
6.6 Contribution in the selectivity of the OS/IS and control of nonspecific interactions
6.7 Specificity toward structural analogs
6.8 Reusability, regeneration
6.9 Conclusion
List of abbreviations
References
7 -
Metal-selective sorbents
7.1 Introduction
7.1.1 Metals
7.1.2 Basic terms used in sorption science and technology
7.2 Sorbents for sequestering metal ions from aqueous solutions
7.2.1 Biomass, industrial and agricultural by-products and wastes
7.2.2 Activated carbons
7.2.3 Mineral adsorbents
7.2.3.1 Clay minerals, zeolites, silica minerals
7.2.3.2 Modified mineral adsorbents
7.2.4 Synthetic sorbents
7.2.4.1 Inorganic synthetic sorbents
7.2.4.1.1 Silica gel
7.2.4.1.2 Metal oxides (e.g., aluminum oxides, magnesium oxides, ferric oxides, manganese oxides, titanium oxides, cerium oxides)
7.2.4.1.3 Aluminosilicates
7.2.4.1.4 Cyanoferrates and hexacyanoferrates
7.2.4.1.5 Zeolites
7.2.4.2 Organic synthetic sorbents
7.2.4.2.1 Ion-exchange resins
7.2.4.2.2 Chelating resins
7.2.4.3 Hybrid sorbents
7.3 Conclusions
References
8 -
Molecularly imprinted polymers
8.1 Introduction
8.2 Preparation of molecularly imprinted polymers
8.2.1 General considerations
8.2.1.1 Template and monomer(s)
8.2.1.2 Crosslinker
8.2.2 Optimization of MIP formulations
8.2.2.1 Computational approach
8.2.2.2 Combinatorial approach
8.2.2.3 Polymerization strategies
8.2.2.4 Template bleeding
8.3 Molecularly imprinted solid-phase extraction (MISPE)
8.3.1 Off-line protocols
8.3.2 Online protocols
8.3.3 In-line protocols
8.3.4 Improved batch protocols
8.4 Selected applications
8.5 Conclusions
References
9 -
Magnetic nanoparticle sorbents
9.1 Introduction
9.2 Preparation techniques
9.2.1 Synthesis of magnetic nanoparticles
9.2.2 Surface modification of magnetic nanoparticles
9.2.2.1 Modification with inorganic materials
9.2.2.2 Modification with small organic molecules
9.2.2.2.1 Surfactant modification
9.2.2.2.2 Silane coupling reagent modification
9.2.2.2.3 Ionic liquid modification
9.2.2.3 Organic polymer modification
9.3 Characterization methods
9.3.1 Crystal structure analysis
9.3.2 Morphological characterization
9.3.2.1 Electron microscopy
9.3.3 Elemental analysis
9.3.3.1 Electron spectroscopy
9.3.4 Surface area analysis
9.3.5 Spectral analysis
9.3.5.1 Fourier transform infrared spectroscopy
9.3.5.2 Absorption spectroscopy
9.3.6 Thermal gravimetric analysis (TGA)
9.3.7 Zeta potential
9.3.8 Magnetism
9.4 Magnetic carbon materials
9.4.1 Magnetic carbon nanotubes (CNTs)
9.4.2 Magnetic graphene and graphene oxide adsorbents
9.4.3 Magnetic porous carbon
9.5 Magnetic metal and metal oxides
9.6 Magnetic metal-organic frameworks
9.6.1 Embedding method
9.6.2 Encapsulation method
9.6.3 Layer-by-layer self-assembly
9.6.4 Physical mixing
9.6.5 Postmagnetization
9.6.6 Other methods
9.7 Magnetic porous organic polymers
9.7.1 Physical mixing
9.7.2 One-pot synthesis
9.7.3 In situ growth synthesis
9.7.4 Postmagnetization and chemical bridging
9.8 Others magnetic nanoparticle sorbents
9.8.1 Magnetic mesoporous materials
9.8.2 Magnetic molecular/ion imprinting polymer
9.8.3 Magnetic restricted access materials
References
10. Metal-organic frameworks
10.1 Introduction
10.2 IRMOF in solid-phase extraction
10.2.1 MOF-5-based sorbent for SPE
10.2.1.1 Directly used as sorbent substrate
10.2.1.1.1 Preparation of Cu/MOF-5 bar
10.2.1.1.2 Headspace microextraction procedure
10.2.1.1.3 Application
10.2.1.2 MOF-5 composite materials for extraction
10.2.1.2.1 Preparation of Fe3O4/MOF-5 composite
10.2.1.2.2 Extraction procedure
10.2.1.2.3 Application
10.2.2 MOF-199-based sorbent for SPE
10.2.2.1 Preparation of MOF-199/CNTs coated fibers
10.2.2.2 Solid-phase microextraction procedure
10.2.2.3 Application
10.3 MIL in solid-phase extraction
10.3.1 MIL-100-based sorbent for SPE
10.3.1.1 Preparation of Fe3O4/MIL-100 composite
10.3.1.2 Automated magnetic dispersive micro-solid-phase extraction (automatic M-D-μSPE) extraction procedure
10.3.1.3 Application
10.3.2 MIL-101-based sorbent for SPE
10.3.2.1 Preparation of MIL-101(Cr)-based microcolumn
10.3.2.2 The online micro-solid-phase extraction procedure
10.3.2.3 Applications
10.4 ZIF in solid-phase extraction
10.4.1 ZIF-8-based sorbent for SPE
10.4.1.1 Preparation of ZIF-8 coated SPME fibers
10.4.1.2 Headspace solid-phase microextraction procedure
10.4.1.3 Application
10.5 UiO in solid-phase extraction
10.5.1 UiO-66-based sorbent for SPE
10.5.1.1 Preparation of UiO-66-based sorbent
10.5.1.2 Magnetic solid-phase extraction
10.5.1.3 Application
10.6 Conclusions
List of abbreviations
References
11 -
Electrospun nanofibers
11.1 Introduction
11.2 Electrospinning process
11.2.1 Polymer solution
11.2.2 Processing conditions
11.2.3 Ambient parameters
11.3 Characterization of electrospun nanofibers
11.3.1 Physical fiber parameters
11.3.1.1 Fiber diameter and size distribution
11.3.1.2 Porosity and topography
11.3.2 Chemical fiber parameters
11.3.2.1 Elemental analysis
11.3.2.2 Chemical bonding
11.3.3 Thermal stability
11.4 Electrospun nanofibers types
11.4.1 Molecularly imprinted polymers (MIPs)
11.4.2 Core-shell and hollow fibers
11.4.3 Polymeric nanofibers
11.4.4 Copolymers
11.4.5 Carbon fibers
11.4.6 Inorganic fibers
11.4.7 Composites
11.4.7.1 Polymer matrix composite nanofibers (PMCNs)
11.4.7.2 Ceramic matrix composite nanofibers (CMCNs)
11.4.7.3 Carbon matrix composite nanofibers (CAMCNs)
11.4.8 Three-dimensional (3D) electrospun nanofibers
References
12 -
Particle loaded membranes
12.1 Introduction
12.2 Membranes modified with nanoparticles
12.2.1 Hollow fiber membranes
12.2.2 Planar membranes
12.3 Particle-loaded membranes in a thin film microextraction format
12.3.1 Nanoparticle-loaded membranes synthesized via electrospinning
12.3.1.1 One pot synthesis of electrospun fibers with embedded nanoparticles
12.3.1.2 Chemical modification to immobilize nanoparticles in electrospun fibers
12.3.1.2.1 Nanoparticles in paper-based coated sorptive phases
12.4 Conclusions
Acknowledgments
References
13 -
Fabric phase sorptive extraction: a new genration, green sample preparation approach
13.1 Introduction
13.2 Building blocks of fabric phase sorptive extraction membranes and their role in extraction
13.3 Preparation of sol-gel sorbent coated fabric phase sorptive extraction membranes
13.3.1 Selection and pretreatment of fabric phase sorptive extraction membrane
13.3.2 Design and preparation of sol solution for sol-gel sorbent coating on the substrate
13.3.3 Sol-gel sorbent coating process using immersion coating technology
13.3.4 Aging, conditioning, and cleaning of sol-gel sorbent-coated fabric phase sorptive extraction membrane
13.3.5 Cutting the FPSE membrane into appropriate size
13.4 Sol-gel sorbents for fabric phase sorptive extraction
13.5 Characterization of fabric phase sorptive extraction membranes
13.6 Working principle of fabric phase sorptive extraction
13.7 Fabric phase sorptive extraction protocol
13.8 Fabric phase sorptive extraction method development
13.9 Advantages of fabric phase sorptive extraction over conventional sorbent-based sample preparation techniques
13.10 Different implementations of fabric phase sorptive extraction
13.10.1 Stir fabric phase sorptive extraction
13.10.2 Stir bar-fabric phase sorptive extraction (stir bar-FPSE)
13.10.3 Magnet integrated fabric phase sorptive extraction (MI-FPSE)
13.10.4 Dynamic fabric phase sorptive extraction
13.10.5 Fabric phase sorptive extraction interfacing with ion mobility spectrometry
13.10.6 Automated online fabric phase sorptive extraction
13.11 Applications of fabric phase sorptive extraction
13.12 Conclusions
References
14 -
In-tube solid-phase microextraction
14.1 Introduction
14.2 Theoretical considerations
14.3 Preparation of designed capillary columns
14.3.1 Polymerization
14.3.2 Electrodeposition
14.3.3 Direct chemical modification
14.3.4 IT-SPME monolithic capillary columns
14.3.5 Fiber IT-SPME
14.3.6 Packed IT-SPME
14.4 Online coupling to liquid chromatography
14.4.1 Draw-eject mode IT-SPME
14.4.2 Flow-through IT-SPME
14.4.3 Miniaturized LC systems
14.5 Off-line development
14.6 Applications
14.7 Conclusions
Acknowledgements
References
15.-
Needle extraction device
15.1 Introduction
15.2 Construction of a needle extraction device
15.3 Determination of VOC in gas samples
15.3.1 Extraction/desorption of VOCs
15.3.2 Use of needle extraction devices for gas analysis
15.4 Determination of VOCs in water samples
15.4.1 Extraction of VOCs
15.4.2 Use of needle extraction devices for water analysis
15.5 Conclusions
References
16 -
Micro-solid-phase extraction
16.1 Introduction
16.1.1 Features of membrane-based μ-SPE
16.2 Sorbents used in μ-SPE
16.2.1 Conventional sorbents
16.2.2 Carbon-based sorbents
16.2.3 Zeolites, silica, and metal-organic frameworks
16.2.4 Biopolymeric materials, inclusion compounds, and molecularly imprinted polymers
16.2.5 Miscellaneous sorbents
16.3 Enhanced μ-SPE: Combination with other microextraction procedures
16.4 Conclusion and future trends
References
17 -
Microextraction by packed sorbent (MEPS) and monolithic packed pipette tips for 96-well plates
17.1 Introduction
17.2 Microextraction by packed sorbent (MEPS)
17.2.1 Effective factors on MEPS performance
17.2.2 MEPS sorbents
17.2.3 MEPS off- and online application
17.2.4 MEPS fields of application
17.2.5 MEPS challenges and future aspects
17.3 Monolithic packed 96-tips
17.3.1 Preparation of monolithic packed tips
17.3.1.1 Treatment of pipette surface
17.3.1.2 Manufacture of monolithic pipettes
17.3.2 Factors affecting the performance of packed 96-tips
17.3.3 Modified monoliths phases for packed pipette tips
17.3.4 Monolithic packed tips applications
17.4 Concluding remarks
References
Further reading
18 -
Stir-bar sorptive extraction
18.1 Theory of stir-bar sorptive extraction
18.2 Method development
18.2.1 Selection of the extraction phase
18.2.2 Selection of the sampling mode
18.2.3 Optimization of extraction parameters
18.2.4 Selection of the desorption mode
18.2.5 Derivatization
18.3 Calibration and method validation
18.4 Application of stir-bar sorptive extraction
18.4.1 Environmental analysis
18.4.2 Food analysis
18.4.3 Biomedical analysis
18.4.4 As a passive sampler
18.5 Conclusions
References
19 -
Matrix solid phase dispersion
19.1 Introduction
19.2 General procedure, traditional sorbents, and new materials
19.3 Extraction of natural components of plants and fruits by MSPD
19.4 MSPD methods for the extraction of contaminants from different matrices
19.4.1 Assisted MSPD extraction and combination with other extraction techniques
19.5 Conclusion
References
20 -
Solid-phase analytical derivatizations
20.1 Introduction
20.2 Solid-phase analytical derivatizations
20.3 Solid-phase analytical derivatization of organic acids
20.3.1 Phenols
20.3.1.1 Anion exchange resin and solid-phase extraction cartridges
20.3.1.2 SPME on-fiber derivatizations
20.3.1.3 Derivatizing on a poly styrene–divinylbenzene crosslinked macroreticular resins
20.3.2 Carboxylic acids
20.4 Solid-phase analytical derivatization of aldehydes and ketones
20.5 Solid-phase analytical derivatization of amines
20.6 Conclusions
References
21 -
Automated and high-throughput extraction approaches
21.1 Introduction
21.2 Evolution of automated SPE approaches toward high-throughput sample extraction
21.2.1 Automated off-line SPE approach for high-throughput extraction
21.2.1.1 Selected illustrative example
Adding IS into the mixing plate (Position 2)
Adding plasma samples into the mixing plate (B)
Conditioning the SPE plate (C)
SPE extraction
Reconstitution of the samples
21.2.2 Online SPE approach for high-throughput sample extraction
21.2.2.1 Selected illustrative example
Conditioning
Equilibrium
Extraction
Cartridge wash
Analyte elution
Clamp washes
21.2.3 Dispersive pipette extraction (DPX) approach for high-throughput sample extraction
21.2.3.1 Selected illustrative example
Preparation of the urine samples and reagents
Conditioning of DPX tips
Loading Samples on DPX tips
DPX wash
DPX elution
21.3 Conclusions
References
22 - Design of experiments and method development
22.1 Introduction
22.2 Methodology of design of experiments
22.2.1 Screening designs for solid phase extraction
22.2.1.1 Full factorial design
22.2.1.2 Fractional factorial designs
22.2.1.3 Plackett-Burman design
22.2.2 Optimization designs and applications for solid phase extraction
22.2.2.1 Central composite design
22.2.2.2 Box-Behnken design
22.2.2.3 Doehlert design
22.3 Conclusions
References
23 -
Environmental applications (water)
23.1 Introduction
23.2 Volatile compounds
23.3 Hydrophobic compounds
23.4 Moderately polar compounds
23.5 Highly polar compounds
Acknowledgments
References
24 -
Environmental applications (air)
24.1 Solid phases for air environmental applications
24.1.1 Active sampling
24.1.2 Passive sampling
24.2 Environmental applications
24.2.1 Rural air monitoring
24.2.2 Urban and industrialized air monitoring
24.2.3 Indoor applications
24.2.4 Applications in occupational exposure
24.3 Conclusions
References
25 -
Solid-phase extraction in bioanalytical applications
25.1 Introduction
25.2 Conventional SPE approches in bioanalytical application
25.3 High throughput sample analysis using 96-well SPE microplates in bioanalytical applications
25.4 Microextraction by packed sorbents (MEPS) in bioanalytical applications
25.5 General conclusions and future perspectives
Acknowledgments
References
Index
A
B
C
D
E
F
G
H
I
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Z

Citation preview

Handbooks in Separation Science The goal of the series and volume editors is to develop a new vehicle for collating, interpreting, and disseminating the essential fundamental and practical information of separation science for future generations of separation scientists and to do this by creating the seminal work in the field. Each volume is designed to cover a specific topic and contains relatively succinct chapters with a sharp focus and clear presentation contributed by leading scientists in the field. The target audience for these volumes is professional scientists with responsibility for managing or participating in research projects in either academia or industry. Included in this group are graduate students and professionals in disciplines other than separation science seeking insight into a topic at a level associated with current capabilities. The current volume follows on from the success of earlier volumes with additional volumes in production or planned for the future. 2012dC.F. Poole (Editor). Gas Chromatography 2013dS. Fanali, P.R. Haddad, C.F. Poole, P. Schoenmakers, D. Lloyd (Editors). Liquid Chromatography: Fundamentals and Instrumentation S. Fanali, P.R. Haddad, C.F. Poole, P. Schoenmakers, D. Lloyd (Editors). Liquid Chromatography: Applications 2015dC.F. Poole (Editor). Instrumental Thin-Layer Chromatography A. Gorak, E. Sorensen (Editors). Distillation: Fundamentals and Principles A. Gorak, H. Schoenmakers (Editors). Distillation: Operation and Applications A. Gorak, Z. Olujic (Editors). Distillation: Equipment and Processes 2017dC.F. Poole (Editor). Supercritical Fluid Chromatography S. Fanali, P.R. Haddad, C.F. Poole, M.-L. Riekkola (Editors). Liquid Chromatography: Fundamentals and Instrumentation, Second Edition S. Fanali, P.R. Haddad, C.F. Poole, M.-L. Riekkola (Editors). Liquid Chromatography: Applications, Second Edition C.F. Poole (Editor). Capillary Electromigration Separation Methods 2018dA.F. Ismail, M.A. Rahman, M.H.D. Othman, T. Matsuura (Editors). Membrane Separation Principles and Applications: From Material Selection to Mechanisms and Industrial Uses 2019dC.F. Poole (Editor). Liquid-Phase Extraction

Solid-Phase Extraction

Edited by

Colin F. Poole

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Contributors

Abbi Abdel-Rehim Faculty of Science and Engineering, University of Manchester, Manchester, United Kingdom Mohamed Abdel-Rehim Department of Clinical Neuroscience, Center for Psychiatry Research, Karolinska Institutet and Stockholm County Council, Stockholm, Sweden Martha B. Adaime Laboratory of Pesticide Residue Analysis (LARP), Chemistry Department, Federal University of Santa Maria, Santa Maria, Rio Grande do Sul, Brazil Hossam Al-Suod Department of Environmental Chemistry and Bioanalytics, Faculty of Chemistry, Nicolaus Copernicus University, Torun, Poland; Centre for Modern Interdisciplinary Technologies, Nicolaus Copernicus University, Torun, Poland Beatriz Albero Departamento de Medio Ambiente y Agronomía, Instituto Nacional de Investigaci
on y Tecnología Agraria y Alimentaria (INIA), Madrid, Spain Tahereh Golzari Aqda Environmental and Bio-Analytical Laboratories, Department of Chemistry, Sharif University of Technology, Tehran, Iran Sergio Armenta Department of Analytical Chemistry, Research Building, University of Valencia, Valencia, Spain María Asensio-Ramos Instituto Volcanol
ogico de Canarias (INVOLCAN), INtech La Laguna, San Crist
obal de La Laguna, Tenerife, Islas Canarias, Espa~na Sara Asgari Environmental and Bio-Analytical Laboratories, Department of Chemistry, Sharif University of Technology, Tehran, Iran Sanka N. Atapattu

CanAm Bioresearch Inc., Winnipeg, MB, Canada

Habib Bagheri Environmental and Bio-Analytical Laboratories, Department of Chemistry, Sharif University of Technology, Tehran, Iran A. Ballester-Caudet Miniaturization and Total Analysis Methods (MINTOTA) Research Group, Departament de Química Analítica, Facultat de Química. Universitat of Valencia, Valencia, Spain

xii

Contributors

Francesc Borrull Department of Analytical Chemistry and Organic Chemistry, Universitat Rovira i Virgili, Tarragona, Spain Bogusław Buszewski Department of Environmental Chemistry and Bioanalytics, Faculty of Chemistry, Nicolaus Copernicus University, Torun, Poland; Centre for Modern Interdisciplinary Technologies, Nicolaus Copernicus University, Torun, Poland P. Campíns-Falc
o Miniaturization and Total Analysis Methods (MINTOTA) Research Group, Departament de Química Analítica, Facultat de Química. Universitat of Valencia, Valencia, Spain S. C
ardenas Departamento de Química Analítica, Instituto Universitario de Investigaci
on en Nanoquímica (IUNAN), Edificio Marie Curie (anexo), Campus de Rabanales, Universidad de C
ordoba, C
ordoba, Spain Beibei Chen

Department of Chemistry, Wuhan University, Wuhan, Hubei, China

Yanlong Chen School of Chemistry, Sun Yat-sen University, Guangzhou, Guangdong, PR China Miguel de la Guardia Department of Analytical Chemistry, Research Building, University of Valencia, Valencia, Spain J
ulia A. de Oliveira Laboratory of Pesticide Residue Analysis (LARP), Chemistry Department, Federal University of Santa Maria, Santa Maria, Rio Grande do Sul, Brazil M.C. Díaz-Li~ n
an Departamento de Química Analítica, Instituto Universitario de Investigaci
on en Nanoquímica (IUNAN), Edificio Marie Curie (anexo), Campus de Rabanales, Universidad de C
ordoba, C
ordoba, Spain Jianwei Dong School of Chemistry, Sun Yat-sen University, Guangzhou, Guangdong, PR China Antonio Martín Esteban Madrid, Spain

Departamento de Medio Ambiente y Agronomía, INIA,

Francesc A. Esteve-Turrillas Department of Analytical Chemistry, Research Building, University of Valencia, Valencia, Spain Nestor Etxebarria Department of Analytical Chemistry, Faculty of Science and Technology, University of the Basque Country (UPV/EHU), Basque Country, Spain; Research Centre for Experimental Marine Biology and Biotechnology (PIE), University of the Basque Country (UPV/EHU), Basque Country, Spain N
uria Fontanals Department of Analytical Chemistry and Organic Chemistry, Universitat Rovira i Virgili, Tarragona, Spain Leon Fuks Centre of Radiochemistry and Nuclear Chemistry, Institute of Nuclear Chemistry and Technology, Warszawa, Poland

Contributors

xiii

Kenneth G. Furton International Forensic Research Institute, Department of Chemistry and Biochemistry, Florida International University, Miami, FL, United States Belén Gonz
alez-Gaya Department of Analytical Chemistry, Faculty of Science and Technology, University of the Basque Country (UPV/EHU), Basque Country, Spain; Research Centre for Experimental Marine Biology and Biotechnology (PIE), University of the Basque Country (UPV/EHU), Basque Country, Spain Department of Chemistry, Wuhan University, Wuhan, Hubei, China

Man He

Irena Herdzik-Koniecko Centre of Radiochemistry and Nuclear Chemistry, Institute of Nuclear Chemistry and Technology, Warszawa, Poland R. Herr
aez-Hern
andez Miniaturization and Total Analysis Methods (MINTOTA) Research Group, Departament de Química Analítica, Facultat de Química. Universitat of Valencia, Valencia, Spain Bin Hu

Department of Chemistry, Wuhan University, Wuhan, Hubei, China

Abuzar Kabir International Forensic Research Institute, Department of Chemistry and Biochemistry, Florida International University, Miami, FL, United States Hian Kee Lee National University of Singapore Environmental Research Institute, National University of Singapore, Singapore, Singapore; NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore, Singapore; Department of Chemistry, National University of Singapore, Singapore, Singapore; Tropical Marine Science Institute, National University of Singapore, Singapore, Singapore Gongke Li PR China

School of Chemistry, Sun Yat-sen University, Guangzhou, Guangdong,

Yanxia Li PR China

School of Chemistry, Sun Yat-sen University, Guangzhou, Guangdong,

A.I. L
opez-Lorente Departamento de Química Analítica, Instituto Universitario de Investigaci
on en Nanoquímica (IUNAN), Edificio Marie Curie (anexo), Campus de Rabanales, Universidad de C
ordoba, C
ordoba, Spain R. Lucena Departamento de Química Analítica, Instituto Universitario de Investigaci
on en Nanoquímica (IUNAN), Edificio Marie Curie (anexo), Campus de Rabanales, Universidad de C
ordoba, C
ordoba, Spain Faranak Manshaei Environmental and Bio-Analytical Laboratories, Department of Chemistry, Sharif University of Technology, Tehran, Iran Rosa M. Marcé Department of Analytical Chemistry and Organic Chemistry, Universitat Rovira i Virgili, Tarragona, Spain

xiv

Contributors

Mohammad Mahdi Moein Department of Clinical Neuroscience, Center for Psychiatry Research, Karolinska Institutet and Stockholm County Council, Stockholm, Sweden Y. Moliner-Martinez Miniaturization and Total Analysis Methods (MINTOTA) Research Group, Departament de Química Analítica, Facultat de Química. Universitat of Valencia, Valencia, Spain C. Molins-Legua Miniaturization and Total Analysis Methods (MINTOTA) Research Group, Departament de Química Analítica, Facultat de Química. Universitat of Valencia, Valencia, Spain Rosa Montes Department of Analytical Chemistry, Nutrition and Food Science, IIAA e Institute for Food Analysis and Research, Universidade de Santiago de Compostela, Santiago de Compostela, Spain Aline L.H. M€ uller Laboratory of Pesticide Residue Analysis (LARP), Chemistry Department, Federal University of Santa Maria, Santa Maria, Rio Grande do Sul, Brazil Nyi Nyi Naing National University of Singapore Environmental Research Institute, National University of Singapore, Singapore, Singapore ~ez Department of Chemical Engineering and Analytical Chemistry, UniOscar N
un versity of Barcelona, Barcelona, Spain; Serra H
unter Fellow, Generalitat de Catalunya, Barcelona, Spain Maitane Olivares Department of Analytical Chemistry, Faculty of Science and Technology, University of the Basque Country (UPV/EHU), Basque Country, Spain; Research Centre for Experimental Marine Biology and Biotechnology (PIE), University of the Basque Country (UPV/EHU), Basque Country, Spain Rosa Ana Pérez Departamento de Medio Ambiente y Agronomía, Instituto Nacional de Investigaci
on y Tecnología Agraria y Alimentaria (INIA), Madrid, Spain Valérie Pichon Department of Analytical, Bioanalytical Sciences and Miniaturization, UMR, CBI, ESPCI Paris, PSL Research University, Paris, France; Sorbonne University, Paris, France Colin F. Poole United States

Department of Chemistry, Wayne State University, Detroit, MI,

Osmar D. Prestes Laboratory of Pesticide Residue Analysis (LARP), Chemistry Department, Federal University of Santa Maria, Santa Maria, Rio Grande do Sul, Brazil Ailette Prieto Department of Analytical Chemistry, Faculty of Science and Technology, University of the Basque Country (UPV/EHU), Basque Country, Spain; Research Centre for Experimental Marine Biology and Biotechnology (PIE), University of the Basque Country (UPV/EHU), Basque Country, Spain

Contributors

xv

Maria Eugênia C. Queiroz Departamento de Química, Faculdade de Filosofia Ciências e Letras de Ribeir~ao Preto, Universidade de S~ao Paulo, Ribeir~ao Preto, SP, Brasil José Benito Quintana Department of Analytical Chemistry, Nutrition and Food Science, IIAA e Institute for Food Analysis and Research, Universidade de Santiago de Compostela, Santiago de Compostela, Spain María Ramil Department of Analytical Chemistry, Nutrition and Food Science, IIAA e Institute for Food Analysis and Research, Universidade de Santiago de Compostela, Santiago de Compostela, Spain Omid Rezvani Environmental and Bio-Analytical Laboratories, Department of Chemistry, Sharif University of Technology, Tehran, Iran Rosario Rodil Department of Analytical Chemistry, Nutrition and Food Science, IIAA e Institute for Food Analysis and Research, Universidade de Santiago de Compostela, Santiago de Compostela, Spain
Miguel Angel Rodríguez-Delgado Departamento de Química, Unidad Departamental de Química Analítica, Facultad de Ciencias, Universidad de La Laguna (ULL). Avenida Astrofísico Francisco S
anchez, San Crist
obal de La Laguna, Tenerife, Espa~ na Ruth Rodríguez-Ramos Departamento de Química, Unidad Departamental de Química Analítica, Facultad de Ciencias, Universidad de La Laguna (ULL). Avenida Astrofísico Francisco S
anchez, San Crist
obal de La Laguna, Tenerife, Espa~na Jack M. Rosenfeld Department of Pathology and Molecular Medicine, McMaster University, Hamilton, ON, Canada Yoshihiro Saito Department of Applied Chemistry and Life Science, Toyohashi University of Technology, Toyohashi, Aichi, Japan
Alvaro Santana-Mayor Departamento de Química, Unidad Departamental de Química Analítica, Facultad de Ciencias, Universidad de La Laguna (ULL). Avenida Astrofísico Francisco S
anchez, San Crist
obal de La Laguna, Tenerife, Espa~na Javier Saurina Department of Chemical Engineering and Analytical Chemistry, University of Barcelona, Barcelona, Spain Sonia Sentellas Department of Chemical Engineering and Analytical Chemistry, University of Barcelona, Barcelona, Spain B
arbara Socas-Rodríguez Departamento de Química, Unidad Departamental de Química Analítica, Facultad de Ciencias, Universidad de La Laguna (ULL). Avenida Astrofísico Francisco S
anchez, San Crist
obal de La Laguna, Tenerife, Espa~na Israel D. Souza Departamento de Química, Faculdade de Filosofia Ciências e Letras de Ribeir~ao Preto, Universidade de S~ao Paulo, Ribeir~ao Preto, SP, Brasil

xvi

Contributors

Małgorzata Szultka-Mły
nska Department of Environmental Chemistry and Bioanalytics, Faculty of Chemistry, Nicolaus Copernicus University, Torun, Poland José L. Tadeo Departamento de Medio Ambiente y Agronomía, Instituto Nacional de Investigaci
on y Tecnología Agraria y Alimentaria (INIA), Madrid, Spain Sze Chieh Tan NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore, Singapore; Department of Chemistry, National University of Singapore, Singapore, Singapore Esther Turiel Spain

Departamento de Medio Ambiente y Agronomía, INIA, Madrid,

Ikuo Ueta Department of Applied Chemistry, University of Yamanashi, Kofu, Yamanashi, Japan Aresatz Usobiaga Department of Analytical Chemistry, Faculty of Science and Technology, University of the Basque Country (UPV/EHU), Basque Country, Spain; Research Centre for Experimental Marine Biology and Biotechnology (PIE), University of the Basque Country (UPV/EHU), Basque Country, Spain J. Verd
u-Andrés Miniaturization and Total Analysis Methods (MINTOTA) Research Group, Departament de Química Analítica, Facultat de Química. Universitat of Valencia, Valencia, Spain Ling Xia School of Chemistry, Sun Yat-sen University, Guangzhou, Guangdong, PR China Renato Zanella Laboratory of Pesticide Residue Analysis (LARP), Chemistry Department, Federal University of Santa Maria, Santa Maria, Rio Grande do Sul, Brazil Shakiba Zeinali Environmental and Bio-Analytical Laboratories, Department of Chemistry, Sharif University of Technology, Tehran, Iran Naiyu Zheng Department of Bioanalytical Sciences, Bristol-Myers Squibb Company, Princeton, NJ, United States Olatz Zuloaga Department of Analytical Chemistry, Faculty of Science and Technology, University of the Basque Country (UPV/EHU), Basque Country, Spain; Research Centre for Experimental Marine Biology and Biotechnology (PIE), University of the Basque Country (UPV/EHU), Basque Country, Spain

Core concepts and milestones in the development of solid-phase extraction

1

Colin F. Poole Department of Chemistry, Wayne State University, Detroit, MI, United States

1.1

Introduction

The origins of solid-phase extraction are as old as chromatography, which in its early days was exploited for the isolation of compounds from mixtures by their selective interaction with a solid stationary phase and subsequent recovery by elution in a mobile phase. Chromatography and extraction have since diverged in their general function in chemical analysis and are regarded as complementary techniques today. Extraction is typically employed for isolation, preconcentration, matrix simplification, or solvent exchange ahead of the separation and identification of compounds by chromatographic-based (and other) techniques. The key to understanding the relationship between these common laboratory techniques is to consider extraction as an enabling technique that modifies sample properties to facilitate a successful separation and detection of target compounds by the most appropriate technique. In the absence of an extraction step the sample would appear to be too complex, too dilute or incompatible with sustaining instrument performance rendering the analysis unsuccessful. Overtime the scale, speed, material costs, and level of automation for the extraction step have adapted to changing laboratory needs. Thus, solid-phase extraction, one variant of extraction methods, is a dynamic field, and while old, it is still heavily researched with the flux of advances far from concluded. At the time of writing, it is reasonable to identify miniaturization, advances in material science, ease of automation, and compatibility with the goals of green analytical chemistry as the primary driving forces maintaining the general interest in advancing the techniques of solidphase extraction [1e3].

1.2

First generation formats

Solid-phase extraction is based on the transfer of target compounds in a gas, liquid, or supercritical fluid matrix to a solid sorbent [4]. Typically, the sample containing the target compounds flows over the solid sorbent which retains the compounds by their favorable interactions with the sorbent. The sorbent is subsequently separated from the sample and the target compounds recovered by solvent displacement or thermal desorption into the gas phase. An early application of solid-phase extraction in the Solid-Phase Extraction. https://doi.org/10.1016/B978-0-12-816906-3.00001-7 Copyright © 2020 Elsevier Inc. All rights reserved.

2

Solid-Phase Extraction

1950s was the use of activated carbon-filled columns to isolate organic contaminants from surface waters for toxicity evaluation [5]. The low concentration of contaminants and the poor capability of instrumental methods to identify compounds and assess their toxicity at that time resulted in large-scale operations in which thousands of liters of water were sampled over several days. The introduction of macroreticular porous polymers in the early 1970s was responsible for redirecting interest in solid-phase extraction for both field and laboratory applications as well as extending its scope to air sampling and the isolation of drugs from biological fluids. These sorbents had reasonable mechanical strength, a large surface area, a large sample capacity, low water retention, and provided high recovery of target compounds by solvent or thermal desorption. Compared with activated carbon the overall recovery of target compounds was generally better and irreversible adsorption and catalytic activity greatly diminished. These properties together with further improvements in instrumental methods facilitated a general downsizing of sorbent beds, a reduction in sample size, and increasing use of solid-phase extraction as a general laboratory technique for a wider range of applications than was previously the case [6,7]. Porous polymers of high thermal stability and low water retention were responsible for revolutionizing the analysis of volatile organic compounds in air and purge gas samples from dynamic stripping of volatile organic compounds from aqueous solution. Compounds trapped on the sorbent bed were thermally desorbed directly into a gas chromatograph for analysis eventually leading to fully automated sampling and analysis systems for routine use [8,9]. The general acceptance of solid-phase extraction for sampling liquids, however, occurred later in the early 1980s with the introduction of disposable cartridge devices containing silica-based chemically bonded sorbents of a suitable particle size for sample processing by gently suction [10e14]. Within a few years cartridge-based solidphase extraction was considered a suitable alternative to liquid-liquid extraction for many applications and entered a period of evolutionary change. Typical cartridge devices consist of short columns (generally an open syringe barrel) containing sorbent with a nominal particle size between 20 and 60 mm, preferably with a narrow particle size range, packed between porous plastic or metal frits, Fig. 1.1. A wide range of sorbent chemistries (silica-based chemically bonded, mixed mode, porous polymer, restricted access media, molecularly imprinted polymers, immunosorbent, bonded cryptands, etc.) are available today providing for the diverse application base of modern cartridge-based solid-phase extraction [12e14]. Low-volume cartridges or precolumn devices soon appeared as the basis of online integrated systems for automation of the sampling and separation processes, in for example, solid-phase extraction (SPE)-liquid chromatography (LC), SPE-gas chromatography (GC), SPE-capillary electrophoresis (CE), SPE inductively coupled plasma spectroscopy (ICP) and LC-SPE-nuclear magnetic resonance spectroscopy (NMR). By the mid-1990s these systems had matured into robust practical systems in use in many laboratories with a high sample workload and little variation in sample matrix, for example, drugs in biological fluids, contaminants in surface waters, target compounds in food extracts, etc. [15e18]. Standard solid-phase extraction procedures lend themselves to automation using robotic platforms or special purpose processing units that simultaneously extract and prepare samples for separation [12,19]. Multiwell plates with a sorbent

Core concepts and milestones in the development of solid-phase extraction

3

Figure 1.1 Solid-phase extraction using a cartridge device for liquid samples (A) and gas-phase samples (B).

bed at the bottom of the well combined with liquid handling robots are suitable for high-throughput parallel sample processing [20]. Cartridge-based solid-phase extraction was initially marketed as a replacement for traditional liquid-liquid extraction. Traditional liquid-liquid extraction methods were viewed as labor intensive, difficult to automate, and frequently plagued by practical problems, such as emulsion formation. In addition, they tend to consume large volumes of high purity solvents, some of which are considered significant health hazards with high disposal costs [5,12,21]. Cartridge-based solid-phase extraction procedures, on the other hand, were more economical, afforded shorter sample processing times, consumed less solvent, and allowed for simpler sample handling procedures. They are also more convenient for field sampling because they minimize the transport and storage problems associated with bulk samples, which have to be returned to the laboratory for processing. These arguments, although persuasive at the time, are not as true today. Modern liquid-liquid extraction procedures in their several forms, known collectively as liquid phase microextraction, address many of the problems associated with traditional liquid-liquid extraction and compete effectively and complement solid-phase extraction procedures [22e24]. It should not be overlooked that cartridge-based solid-phase extraction has its own, albeit different, problems to those of liquid-liquid extraction. The extraction properties of solid-phase sorbents are not as reproducible as those for solvents. Typical sorbent surfaces contain more than one functional group in amounts not easily replicated by synthesis. In addition, their sorption properties are typically more adversely affected by contaminants. Sorbents also tend to have a higher level of extractable contaminants originating from the manufacturing process and from packaging materials. This chemical background may interfere in the subsequent sample analysis. Rinsing of the sorbent bed prior to use and using blanks to establish background contamination levels diminishes sample throughput and adds significantly to solvent consumption

4

Solid-Phase Extraction

and sample processing costs. Sorbents are also affected by sample processing conditions, such as overloading, displacement of target compounds by excess matrix, and blocking of sorbent pores. These problems easily go unnoticed resulting in unforeseen changes in the recovery of target compounds. Solid-phase microextraction (SPME) was introduced in 1990 as an alternative approach to cartridge-based solid-phase extraction, which was well established by this time [3,25]. It is sometimes thought of as a miniaturized version of solidphase extraction as implied by its name, but this is not the case. The downscaling of solid-phase extraction to accommodate small sample volumes, but otherwise incorporating the same extraction principles, is correctly known as porous membrane protected micro-solid-phase extraction, or simply micro-solid-phase extraction (mSPE) and first appeared in 2006 [26,27]. It employs a small sorbent bed enclosed in a porous membrane allowing solvent and low-mass compounds access to the sorbent bed while excluding macromolecules. The ratio of sample volume to sorbent surface area (or volume) is low in the relative sense but large compared with the fiber format used in solid-phase microextraction, and exhaustive extraction of target compounds remains the general goal. The fiber format employs a thin layer of immobilized extraction phase coated on the outside of a fused-silica or metal-wire support. The extraction fiber is attached to the plunger of a modified microsyringe which both protects and facilitates manipulation of the fiber during sampling, Fig. 1.2 [28]. The main difference between solid-phase microextraction and solid-phase extraction is the extreme ratio of the sample to sorbent volume or surface area. An extremely small amount of sorbent is utilized in comparison to the sample volume in solid-phase microextraction and exhaustive extraction of target compounds is not favored. Typically only a small portion of the target compounds is extracted from the sample (negligible depletion unless the distribution constant is unusually large). This amount increases with the extraction time until equilibrium conditions are reached, as illustrated by Fig. 1.3 [29]. For calibration either the linear portion of the preequilibrium or at or near equilibrium conditions are selected [3,30,31]. The time to reach equilibrium can be long, in which case the linear portion of the preequilibrium curve becomes the preferred choice. Extraction in the preequilibrium region is kinetically controlled and mass transfer is dominated by diffusion (stagnant solution) or by the method of agitation (agitated solution). The selectivity of the extraction process is usually poor in the preequilibrium region compared with equilibrium sampling. The latter is controlled by the differences in the extraction phase-sample solution distribution constants. The small volume of extraction phase favors applications in field sampling (spot and time weighted average), as it is unnecessary to determine the sample volume processed. The amount of extracted target compounds is independent of the sample volume so long as the product of the distribution constant and extraction phase volume (or surface area) is small compared with the sample volume [29,32]. Solid-phase microextraction can be used for sampling by direct immersion, headspace extraction, or extraction with membrane protection [28]. It integrates sampling, extraction, concentration, and sample introduction into a single solvent-free step for gas chromatography and mass spectrometry and with minimal solvent use for liquid chromatography and capillary electrophoresis. Sample processing requires

Core concepts and milestones in the development of solid-phase extraction

5

Figure 1.2 Device for solid-phase microextraction utilizing a fiber format.

two steps: the distribution of target compounds between the extraction phase and the sample matrix and desorption of the extracted compounds by thermal desorption (solventless extraction) or solvent desorption. Only a small volume of solvent is required for solvent desorption due to the small volume of extraction phase. Thus, the concentration of target compounds in the desorption solvent is relatively high even though the amount extracted is considerably less than for exhaustive extraction. The minimal dilution, or complete transfer in the case of gas phase desorption, compensates for the low absolute amount of extracted compounds. The often cited advantages of solid-phase microextraction are its ease of miniaturization, ease of automation, and straightforward coupling with different measurement systems [2,3,33,34]. The main factors affecting sampling efficiency include: the extraction phase chemistry, extraction mode, agitation method, sample modification (pH, ionic strength, presence of organic solvent, etc.), extraction time, and desorption conditions. Limitations include the small number of commercially available extraction phases, the fragile nature of fused-silica fibers, limited fiber reusability due to carryover or matrix contamination, and the short lifetime of physically coated fibers. Problems are more

6

Solid-Phase Extraction

Figure 1.3 Extraction time profile for sampling with a coated fiber (solid-phase microextraction). Amount of analyte extracted ¼ n (ne at equilibrium), Kfs ¼ analyte distribution constant between the extraction phase and sample, Vf ¼ volume of extraction phase, Vs ¼ sample volume, and Cs ¼ analyte concentration in the sample matrix. Reproduced from Souza-silva EA, Jiang R, Rodriguez-Lafuente A, Gionfriddo E, Pawliszyn J. A critical review of the state of the art of solid-phase microextraction of complex matrices I. Environmental analysis. Trends Anal Chem 2015;71:224e235 with permission.

common with direct immersion in samples of high complexity, such as biological and food samples, for which the development of biocompatible fiber coatings is an active research area [35e37].

1.3

Second generation formats

The low packing density typical of cartridge devices results in the use of longer sorbent beds than necessary to compensate for reduced retention due to channeling [38]. The larger bed mass, in turn, increases nonspecific matrix adsorption resulting in an increase in contamination of extracts. The disk format utilizes smaller particles in a more stable packing configuration to minimize channeling, affording a cleaner chemical background through reduced matrix adsorption. Solid-phase extraction disks are available in at least three different formats. Particle-loaded membranes, introduced in 1990, consist of a web of polytetrafluoroethylene (PTFE) microfibrils, suspended in which are sorbent particles of about 8e12 mm in diameter [39,40]. These membranes are flexible with a homogeneous structure containing upwards of 90% (w/w) sorbent particles in the form of circular disks about 0.5 mm thick with diameters from 4 to 96 mm. For general use they are supported on a sintered glass disk (or other support) in a standard filtration apparatus using suction to generate the desired flow through the membrane. Particle-loaded membranes are also available in a syringe barrel (cartridge) format. In this case, the sorbent bed contains particles of a larger diameter, about 50 mm, in thicker disks, about 1.0 mm, sealed into the base of an open syringe barrel. Particle-embedded

Core concepts and milestones in the development of solid-phase extraction

7

glass fiber disks contain 10e30 mm diameter sorbent particles woven into a glass fiber matrix [41]. The small-diameter disks are rigid and self-supporting, while the larger diameter disks require a supporting structure similar to particle-loaded membranes. Laminar disks consist of a sandwich of 10 mm sorbent particles in a consolidated 0.5e1.0 mm bed located between two glass-fiber filters, with a screen to hold the filters in place [10]. The 50 mm diameter laminar disks are typically mounted in an open syringe barrel superficially similar to conventional cartridge devices [10]. The slow sample processing rates for large sample volumes typical of cartridges and their low tolerance to blockage by particles and sorbed matrix components provided the initial interest in disk technology for environmental applications, such as the analysis of surface waters for trace contaminants [40]. On account of their larger cross-sectional area and decreased pressure drop disks allow the use of higher sample flow rates resulting in shorter sample processing times [38]. An integral prefilter attached to the top surface of the disk reduces plugging by suspended particles. Small-diameter disks are suitable for handling samples of restricted volume. Smalldiameter disks facilitate integrated sample processing techniques, such as in-vial desorption and on-disk derivatization [42]. The large surface area per unit bed mass of disks facilitates their use for passive sampling in which the disk is suspended in the sample as opposed to the conventional approach of passing the sample through the disk in a manner similar to filtration. The slow equilibrium of the extraction process, even for agitated solutions, however, is a limitation for laboratory applications but is less of a concern for field studies [43]. Particle-loaded membranes have been utilized for biomimetic extraction as surrogate models for bioconcentration and for toxicity risk assessment [44]. Disk technology contributed directly to the automation of solid-phase extraction methods through the development of multiwell extraction plates, used for the cleanup of samples in high-throughput screening techniques in drug development [20]. The characteristic physical properties of disks have supported their application for integrated sampling/detection techniques such as in situ radiochemical, phosphorescence, and X-ray fluorescence detection and as a substrate for MALDI mass spectrometry [45e47]. Microextraction by packed sorbent bed (MEPS) was introduced in 2004 as a miniaturized version of cartridge-based solid-phase extraction designed for handling smallsample volumes with a view to easy automation using a laboratory liquid handler [48,49]. The extraction device is typically a 100e250 mL syringe housing a short bed containing 1e2 mg sorbent located either between the plunger and the needle or built into the needle, Fig. 1.4. The MEPS syringe facilitates low-dead volume sample processing by vertical movement of the plunger with sample and solvent flow possible in both directions through the sorbent bed in a cyclic fashion. This is in contrast to conventional solid-phase extraction techniques which utilize a unidirectional flow and a single contact between the sample and solvents and the sorbent bed. Needle-based extraction formats now include internally coated needles [50] and needle trap devices with sorbent-packed needles [50,51]. Internally coated needles typically employ a stainless steel needle internally coated with a 50 mm thick film of immobilized stationary phase resulting in an open structure similar to open tubular columns in gas chromatography. Typical applications include automated headspace sampling of

8

Solid-Phase Extraction

Figure 1.4 Modified syringe for microextraction by packed sorbent with an expanded view of the sorbent bed (canister). The canister has a dead volume of about 7 mL. Reproduced from Moein MM, Abdel-Rehim A, Abdel-Rehim M. Microextraction by packed sorbent (MEPS). Trends Anal Chem 2015;67:34e44 with permission.

volatile organic compounds referred to as solid-phase dynamic extraction (SPDE). Needle traps consist of a specially designed stainless steel needle packed with adsorbent. For extraction a fixed volume of a gas or liquid sample is passed through the extraction needle with recovery of target compounds by thermal desorption, or less commonly, by solvent desorption. Needle traps have also been used as passive sampling devices for gas phase samples. Needle traps are more durable than solidphase microextraction fibers and have a higher extraction capacity with the possibility of exhaustive extraction. The extraction rate and sensitivity of fiber-based solid-phase microextraction methods can be enhanced by increasing the volume and/or surface area of the extraction phase. Simply increasing the thickness of the extraction phase in the fiber format results in long extraction times and alternative formats have been explored to tackle this issue resulting in the development of in-tube, stir bar, rotating disk, thin-film, and dispersed particle approaches [2,3,52,53]. An important performance parameter for solid-phase microextraction is the equilibrium time (the time required to reach 95% equilibrium where statistically no difference in the amount extracted is observed by extending the sampling time). Shorter equilibration times can be obtained by utilizing a geometry for the extraction phase with a higher surface area-to-volume ratio than the cylindrical geometry of fiber-based solid-phase microextraction devices. The solid-phase microextraction arrow, Fig. 1.5, consists of a steel

Core concepts and milestones in the development of solid-phase extraction

9

Figure 1.5 Arrow solid-phase microextraction device with exposed sorbent (sampling mode) left and injection (covered) mode right. Reproduced from Helin A, Ronnko T, Parshintser K, Hartonen K, Schillin B, Laubli T, Riekkola ML. Solid-phase microextraction arrow for the sampling of volatile amines in wastewater and atmosphere. J Chromatogr A 2015;1426:56e63 with permission.

rod of larger diameter than a conventional fiber coated with a larger amount of extraction phase, while still being compatible with thermal desorption in a standard injection liner of a gas chromatograph on account of its dimensions and sharp, closed tip [50,54]. Alternatively the surface area-to-volume ratio of the extraction phase can be increased using a thin-film format in which the extraction phase is immobilized on the outer surface of a support of suitable geometry, Fig. 1.6 [29,55,56]. The thin-film geometry is the basis for fully automated parallel sample processing in a

Figure 1.6 In-tube solid-phase microextraction with the extraction column utilized as the sample loop of the injection valve of a liquid chromatograph. The valve is shown in the extraction position (load) on the left and extract injection position (inject) on the right. Reproduced from Queiroz MEC, Melo LP. Selective capillary coating material for in-tube solidphase microextraction coupled to liquid chromatography to determine drugs and biomarkers in biological samples. A review. Anal Chim Acta 2014;826:1e11 with permission.

10

Solid-Phase Extraction

96-well plate format utilizing an extraction unit fashioned into a 96-blade device in which the extraction phase is coated over the flat end of each blade [33,52,55]. An early approach for automated solid-phase microextraction from 1997 was the coupling of an internally coated capillary column, the extraction device, to a liquid chromatograph for separation and detection known as in-tube solid-phase microextraction [57e61]. Two approaches are typically used today [59]. In the first approach a short capillary column is placed between the injection loop and injection needle of an autosampler. The injection syringe repeatedly draws and ejects samples from a series of vials under computer control cycling the sample through the capillary column in the forward and reverse direction for each sample until equilibrium is reached or extraction is sufficient. The extracted compounds are subsequently desorbed by solvent from the extraction column and transported to the analytical column for separation. Alternatively, the capillary column is used as a sample loop for an injection valve and target compounds are extracted from the sample with the valve in the load position and then transferred to the column by mobile phase after switching the valve to the inject position, Fig. 1.6 [59]. This arrangement is favored for handling larger sample volumes to increase sensitivity. Wire-in-tube or fiber-in-tube configurations can be used to enhance the extraction rate and efficiency by reducing the phase ratio of the extraction column. Capillary columns of the type used for gas chromatography are often used as the extraction device, as well as a wider range of laboratory-made extraction phases to enhance selectivity, for example, poly(pyrrole), restricted access media, immunosorbents, molecularly imprinted polymers, monolithic sorbents, etc. [59]. Samples are processed sequentially through in-tube solid-phase microextraction, which cannot be considered high-throughput when compared with parallel sample processing using the 96-well plate format. Other general limitations include a low extraction efficiency for some compounds, a low sorbent loading capacity, instability of some extraction phases, and long extraction times resulting from poor mass transfer kinetics. Stir bar sorptive extraction was introduced in 1999 and quickly gained popularity [62e65]. The extraction device consists of a magnetic stir bar in a glass sleeve externally coated with a 0.3e1.0 mm layer of extraction phase. Liquid samples are analyzed by direct immersion of the stir bar with vigorous stirring and gas phase samples by suspending the stir bar in the headspace above the sample. Extracted compounds are recovered by either thermal desorption or solvent desorption. The limited number of commercially available extraction phases [poly(dimethylsiloxane), poly(acrylate), and an ethylene glycol/silicone copolymer] limit general applications to the extraction of compounds of low- and intermediate-polarity from water [63,65]. The larger volume of extraction phase, typically two orders of magnitude or more compared with fiber-based devices, is responsible for the enhanced extraction efficiency as well as the longer extraction times required to reach either equilibrium or sufficient extraction. The relatively low-selectivity of commercially available extraction phases result in significant matrix sorption with possible interference in the separation process [65]. The sampling process is not easy to automate and a special purpose-designed thermal desorption unit is required for the sequential desorption of extracted compounds from stir bars for gas chromatography [62]. Direct contact between the stir bar and the extraction vessel and the high stirring rates employed for direct immersion extraction

Core concepts and milestones in the development of solid-phase extraction

11

results in a loss of coating due to friction, thus reducing the lifetime of the stir bar. Rotating disk sorbent extraction attempts to address this problem with a novel stir bar design [66e68]. A PTFE disk with an embedded magnetic bar at its base and extraction phase coated only on the top surface is used. An alternative design has a cavity on the top surface loaded with sorbent and covered by a glass fiber filter or semipermeable membrane. There is no contact between the extraction phase and the extraction vessel during stirring prolonging the lifetime of the disk and the protected cavity on the top surface allows a wider choice of sorbent chemistries to be exploited. Further developments based on stir rod and stir membrane devices have been proposed [67]. Nanoparticles have a large surface area-to-volume ratio and magnetic nanoparticles are promising extraction materials for dispersive solid-phase extraction [69e72]. The sorbent material does not need to be packed in a particular type of sampling device, unlike for traditional solid-phase extraction methods. The small particle size allows their dispersion throughout the sample volume to minimize mass transfer problems and their magnetic core facilitates easy collection with a permanent magnet. The efficient dispersion of the sorbent into the matrix yields a high recovery of target compounds limited only by their distribution constants and clean extracts that depends on the ability of the sorbent to reject matrix components. The sequence of steps in the extraction process is illustrated in Fig. 1.7 [70]. Particles are typically 1e100 nm in diameter and made up of several layers, typically with a magnetic core of iron, nickel, cobalt, or any of their oxides. Magnetite (Fe3O4) is the most common magnetic core.

Figure 1.7 Procedural steps for dispersive solid-phase extraction using magnetic nanoparticles. Reproduced from Wlerucka M, Biziuk M. Application of magnetic nanoparticles for magnetic solid-phase extraction in preparing biological, environmental and food samples. Trends Anal Chem 2014;59:50e58 with permission.

12

Solid-Phase Extraction

The magnetic core is typically encased in a shell of inorganic oxide (silica, alumina, manganese dioxide, etc.) or carbon. This can be coated with polymer or the surface modified by chemical reaction to modify selectivity. A large number of surface modified nanoparticles have been synthesized for dispersive solid-phase extraction but only a few are commercially available [71,72]. Nanofibers represent an alternative to nanoparticles for some applications in solidphase extraction [73]. Nanofibers are continuously spun wires with tunable diameters in the nanometer to micrometer range, variable porosity, and large surface area; they are synthesized in the laboratory by applying a high voltage between a viscous solution of a polymer and a collector electrode (in the form of a wire for coated rod devices or sheet of conductive material for membrane devices). A mat of electrospun fibers sealed in a special holder affords a suitable device for conventional solid-phase extraction characterized by a high retention capacity and a low backpressure. Depositing a mat of fibers on the surface of a suitable substrate provides materials for thin-film microextraction. Positioning a nanofiber sheet in the loop of an injection valve affords a suitable interface for in-tube solid-phase microextraction coupled to liquid chromatography. Fiber-coated wires are suitable for solid-phase microextraction. Nanofibers can also be packed into the lower portion of pipette tips, to fashion extraction devices for sampling small volumes. A wide range of single polymers, copolymers, composites, and hybrid nanoparticle loaded fibers have been described so far, but only a few have been used in solid-phase extraction. Limited commercial availability of nanofibers and devices containing nanofibers has restricted their applications to experimental studies. Monoliths provide an alternative bed structure to particle-packed beds for solidphase extraction [74e77]. A monolith is a single rod of biporous organic or inorganic polymer with a uniform structure consisting of flow-through pores (1e2 mm in diameter) providing high permeability (low back pressure) and mesopores providing a high surface area for retention and sample capacity. Organic monoliths are easily prepared in (usually) a one-step process in a suitable mold containing monomers (styrene, acrylate, methacrylate, etc.), cross-linking agent (dimethacrylate, divinylbenzene, etc.), porogen, and a free radical initiator. Inorganic monoliths are mostly based on silica but their preparation is more difficult. They are typically prepared from mixtures of a tetraalkyloxysilane, poly(ethylene glycol), and an acid catalyst forming an initial silica sol that is transformed to a gel by aging. Silica monoliths are more mechanically stable, less prone to changing dimensions (swelling) in different solvents, have a higher surface area, and are easily modified by reaction of surface silanol groups. Silica monoliths are commonly used for the extraction of small molecules in cartridge and disk formats. Organic monoliths are a better choice for the extraction of biopolymers due to their relatively low surface areas, greater biocompatibility, and higher tolerance of extreme pH. Hybrid organic silica monoliths combine the positive features of organic and silica polymer monoliths, but are not commercially available [77]. Monoliths incorporating nanoparticles, molecular-organic frameworks, immunosorbents, molecularly imprinted polymers, aptamers, etc., have been developed to enhance selectivity or sample capacity, but only as experimental materials [76e79]. Wide diameter (4 mm) rod structures became available only recently. The main applications of mainly narrow diameter silica monoliths or thin-film coatings have been

Core concepts and milestones in the development of solid-phase extraction

13

described for in-tube solid-phase extraction, online precolumn liquid chromatography, fiber and stir bar coatings, and microfluidic devices [75,77]. In the disk format monoliths are typically used in pipette tips, spin column (disk solid-phase extraction utilizing centrifugal forces for sample processing), and in multiwell extraction plates [80].

1.4

Sorbent chemistries and properties

Common sorbents for solid-phase extraction can be classified as inorganic oxides, lowspecificity sorbents (silica-based chemically bonded phases, porous polymers, and carbon) and compound and group-selective or high-specificity sorbents (ion exchange, mixed mode, macrocyclic, restricted access, affinity-based, and molecular imprinted polymers) [13]. Alternative classification schemes are possible as well as a wider range of subcategories [1,3,52,81e84]. There is also a considerable disconnect between the enormous number of experimental sorbents described in the contemporary literature and the much smaller number of commercially available sorbents. Outside of research centers and university laboratories the skills, interest, and experience necessary to synthesize experimental sorbents are not usually available. Further details of experimental sorbents can be found in the chapters that follow this general overview. Industrial and regulatory applications are dominated by commercially available sorbents, which are the focus of this section.

1.4.1

Inorganic oxides

The most important inorganic oxide adsorbent for solid-phase extraction is silica gel and to a lesser extent alumina, titania, zirconia, florisil, and diatomaceous earth. Inorganic oxides have a high concentration of active functional groups on their surface responsible for the adsorption of analytes by polar, ion-exchange, and Lewis acid/ base interactions [85]. For aqueous samples ion-exchange and Lewis acid/base interactions dominate while dipole-type and hydrogen-bonding interactions are usually as or more important for nonaqueous samples. Silica gel is in many ways a near ideal adsorbent for the extraction of small polar organic compounds and is available in a wide range of particle sizes for use in different extraction formats, average pore sizes, in the range 4e30 nm, and specific surface areas from 300 to 800 m2/g [5,12]. Silica gel is also a common substrate for monolithic sorbents [77]. The sol-gel process allows silica gel to be utilized as a thin porous coating with chemical attachment to the supporting substrate to increase durability [25,52]. Silica is soluble in aqueous solutions at pH > 7.4 and its use is limited to acidic and neutral solutions. Organic-inorganic hybrid particles can be used to extend the pH operating range of silica-based sorbents [86]. Alumina is available as neutral, acidic, or basic forms determined as an apparent surface pH depending on processing conditions. Typical adsorbents for solid-phase extraction have an average pore size of 6 nm and a specific surface area of 150 m2/g. The surface chemistry of alumina is more complex than silica and is dominated by ion-exchange and Lewis acid/base interactions. Stability to extreme pH favors its use

14

Solid-Phase Extraction

for extraction based on ion-exchange. It also has a high adsorption capacity for metal ions. Titania and zirconia have strong Lewis acid/base sites and are used for the selective isolation of polyoxy anions (phosphates, phosphonates, borates, carboxylates, sulfates, and in particular, phospholipids) [85]. Florisil is a synthetic magnesium silicate with a surface area of about 250e300 m2/g and an apparent surface pH of about 8.5. It is commonly used for sample cleanup in the isolation of pesticide residues from fats and oils. Diatomaceous earth is a flux-calcined form of silica gel with a low surface area. It is mainly used as a filter aid in sample preparation and as a dispersant for solvent extraction (matrix solid-phase dispersion) [87].

1.4.2

Low-specificity sorbents

Low-specificity sorbents for aqueous samples include silica-based chemically bonded sorbents and coatings, porous polymers and polymer coatings, and various forms of carbon [1,10e13]. For gas phase samples porous polymers and various forms of carbon, and occasionally, inorganic oxides or inorganic oxides physically coated with a liquid phase or reactive material are typically used [8,9,88e90]. Low-specificity sorbents are characterized by retention governed by dispersive interactions accompanied by weak or modest polar intermolecular interactions. Silica-based porous particle, monoliths, and thin films with chemically bonded surfaces are prepared by reacting surface silanol groups with organosilanes forming stable siloxane bonds. By varying the silica substrate and organosilane reagent chemically bonded surfaces with a wide range of properties (pore size, bound ligand type, bonding density, etc.) can be prepared by chemistry developed for organosiloxane-bonded silica stationary phases for liquid chromatography [91,92]. Some examples are indicated in Table 1.1. Chemically bonded sorbents are generally synthesized by reaction of monofunctional or trifunctional silanes with silica, followed by endcapping (reaction of sterically crowded silanol groups with a small-size organosilane) in some cases. Monofunctional reagents result in monomeric surface coverage while trifunctional reagents can react both with the silica surface and hydrolyzed reagent forming extended polymeric layers of higher carbon loading and greater pH stability. Chemically bonded sorbents with high surface areas (500e600 m2/g), long alkyl chains, and high phase loading are generally used for the isolation of low-mass compounds from aqueous solution, while wide pore materials with short alkyl chains are used for macromolecules. For silica-based, alkylsiloxane-bonded sorbents retention increases with solute size, and for compounds of a similar size, is reduced by polar interactions with water (particularly hydrogen bonding) and by ionization for aqueous samples. Siloxane-bonded sorbents containing polar functional groups (e.g., 3-cyanopropyl, 3-aminopropyl, and spacer-bonded propanediol) are used mainly to isolate target compounds from samples dissolved in organic solvents based on their selective interactions with polar functional groups of the target compounds. They are generally less retentive than typical alkylsiloxane-bonded sorbents for aqueous samples. Silica-based organosiloxane-bonded sorbents have two general limitations in addition to small, and possibly inadequate, breakthrough volumes for low-mass, highly polar organic compounds. The silica-based sorbents contain a low concentration of

Core concepts and milestones in the development of solid-phase extraction

15

Table 1.1 Structures of porous silica-based, monomeric, organosiloxane-bonded sorbents (hSi-R). Type

Symbol

Functional group

Structure (R)

Alkyl

C30

Tricontane

-C30H61

C18

Octadecyl

-C18H37

C8

Octyl

-C8H17

C4

Butyl

-C4H9

Cyclohexyl

CH

Cyclohexyl

-C6H11

Aromatic

PH

Phenyl

-C6H5

BH

Biphenyl

-C12H9

Cyano

CN

3-Cyanopropyl

-CH2CH2CH2CN

Amino

NH2

3-Aminopropyl

-CH2CH2CH2NH2

Hydroxyl

DIOL

Spaced-bonded propanediol

-CH2CH2CH2OCH2CH(OH) CH2OH

Quaternary amine

SAX

Trimethylaminopropyl

-CH2CH2CH2N(CH3)þ Cl

Carboxylic acid

CBA

Carboxylpropyl

-CH2CH2CH2CO2H

Sulfonic acid

PRS

Propanesulfonic acid

þ -CH2CH2CH2SOe 3 H

SCX

Benzenesulfonic acid

þ -C6H4SO 3 H

ionized silanol groups capable of ion-exchange interactions with basic compounds as well as poor hydrolytic stability at extreme pH (2 > pH > 8) [93,94]. Low recovery of basic compounds can result from the inability of the elution solvents to displace these compounds from the ion-exchange sites (addition of a competing base to the elution solvent might solve this problem). Macroreticular porous polymers are biporous copolymers of styrenedivinylbenzene or acrylic esters, typically, consisting of agglomerates of randomly packed microspheres permeated by a network of holes and channels [91,95]. The highly strained porous structure is swollen to different extents by organic solvents and aqueous-organic solvent mixtures, and is partially responsible for the favorable retention properties of these materials. The addition of small volumes of water-miscible organic solvents, used as a processing aid to minimize dewetting of the polymer during sampling, can have a remarkable effect on the breakthrough volume for these sorbents [96]. Macroreticular porous polymers with surface areas typically less than 600 m2/g, exhibited inadequate retention for low-mass polar compounds, which led to the development of hyper-cross-linked polymers with specific surface areas of 800e2000 m2/g and commensurate increased retention [97,98]. Hydrophilic copolymer sorbents were developed to reduce interfacial tension between the polymer surface and water,

16

Solid-Phase Extraction

increasing the retention of polar compounds by enhancing their contact with the sorbent. They have the added advantage of being fully water wettable simplifying sample processing. These are typically copolymers of N-vinylpyrrolidone and divinylbenzene, methacrylate and divinylbenzene, or surface-modified conventional styrenedivinylbenzene polymers [97]. The main forms of carbon used for solid-phase extraction are activated carbon, graphitized carbon blacks, carbon nanotubes, and porous graphitic carbon [99e104]. Granular activated carbon is prepared by the low temperature oxidation of vegetable charcoals and has a large surface area (300e2000 m2/g), a wide pore distribution and a heterogeneous surface containing active functional groups. Graphitized carbon blacks are (largely) nonporous with moderate surface areas of 5e100 m2/g. Their surfaces are contaminated by oxygen species and other functional groups depending on the origin of the material used for preparation of the carbon sorbent by pyrolysis. The retention mechanism for activated and graphitic carbon is quite complex in which surface functional groups contribute to retention, for example, by ion exchange. Porous graphitic carbon prepared by the silica-template process has a low level of surface contamination but is used less frequently than other forms of carbon because of its relatively high cost. For aqueous samples retention is influenced by hydrophobic forces (increases with solute size), by adsorption on the electronically polarizable surface (dipolar interactions and electron lone pair interactions increase retention), and by contributions related to the microscopically flat surface of graphitic carbon in which planar compounds are retained preferentially over bent and angular compounds [105]. Carbon nanotubes (also cones, disks, horns, fibers) are relatively new materials characterized by a large surface area to volume ratio, high thermal stability, and with a surface that is easily modified to adjust their properties [83,101e104]. Carbon nanofibers have significantly larger dimensions than carbon nanotubes and are preferred for cartridge-based solid-phase extraction [106]. Single-walled carbon nanotubes consist of a single sheet of graphene rolled up in the shape of a cylinder with typical diameters from 0.4 to 3.0 nm and with ends normally capped by fullerenelike structures. Multiwalled carbon nanotubes consist of several rolled up graphene sheets concentrically arranged around a common axis with typical diameters from 1.4 to greater than 100 nm with (usually) capped ends. Carbon nanotubes can be prepared in various lengths up to a few micrometers. Graphene has a high adsorption capacity for compounds of all types including inorganic ions [107]. Surface modification not only enlarges their potential range of applications but equally important, enhances their dispersion in solvents, which otherwise barely occurs due to strong intertube interactions. Experimental materials with a wide range of physically coated and chemically bonded surface functional groups have been described but are not commercially available [103,104]. Graphene is quite reactive and surface modification by halogenation, hydrogenation, oxidation, and radical and nucleophilic addition make it suitable for the preparation of more selective sorbents. Cross-linked poly(siloxanes) analogous to those used as stationary phases for gas chromatography are commonly used as coatings for film-based formats such as fiber, in-tube and thin-film solid-phase microextraction and stir bar sorptive extraction [108,109]. More polar experimental coatings based on copolymers with polar and

Core concepts and milestones in the development of solid-phase extraction

17

hydrogen-bonding monomers, for example, poly(acrylonitrile), poly(pyrrole), poly(urethane), and poly(dopamine) have also been described [59,109e111]. Currently only a limited number of different fiber coatings for solid-phase microextraction is commercially available, notably single polymeric phases such as poly(dimethylsiloxane) and poly(acrylate) and dispersions of solid adsorbents, such as carboxen (carbon molecular sieve) and poly(divinylbenzene) in a supporting polymeric matrix [28,82]. For stir bar sorptive extraction commercially available coatings are limited to poly(dimethylsiloxane), poly(acrylate), and poly(ethylene glycol)-modified silicone [109]. Poly(dimethylsiloxane) coatings meet most demands for the extraction of low and moderately polar compounds from water but are of limited utility for the extraction of polar compounds and ions. Thermal desorption is typically used for recovery of extracted compounds requiring thermally stable coatings, although liquid desorption is possible. Some polymers absorb organic solvents resulting in a change in dimensions as well as stability, limiting applications employing direct immersion. Thin films of poly(dimethylsiloxane) are used as a secondary coating for other sorbents to improve their biocompatibility for sampling by direct immersion [3,35]. Various porous polymers, carbon-based, and inorganic oxide sorbents as well as liquid-coated sorbents are commonly used for extracting volatile organic compounds from air and for work place monitoring [88,89,92,112,113]. Tenax, a polymer based on 2,6-diphenyl-4-phenylene oxide, revolutionized sorbent trapping of semivolatile organic compounds from air and purge gas from aqueous samples (purge-and-trap devices). Tenax differs from most other polymeric sorbents in that it is a granular powder of low surface area, about 35 m2/g, but has exceptional thermal stability, up to about 375 C, facilitating the recovery of low-volatility compounds by thermal desorption. It exhibits strong retention of semivolatile organic compounds (> C7) at room temperature with minimal adsorption of water vapor [114]. Macroreticular porous copolymers, such as poly(styrene-divinylbenzene), have higher surface areas of 300e750 m2/g but significantly lower thermal stability (< 250 C) limiting the mass range of compounds that can be recovered by thermal desorption compared with Tenax. Besides Tenax, carbon in the form of activated carbon, graphitized carbon blacks and carbon molecular sieves are widely used [99,115,116]. Graphitized carbon blacks are primarily used for trapping volatile organic compounds, either separately or in combination with Tenax. Carbon molecular sieves are prepared by the controlled pyrolysis of porous polymers and consist of cross-linked graphite crystallites with a disordered cavity-aperture structure. They are microporous with large surface areas (500 to >1200 m2/g) and are used primarily for trapping volatile organic compounds (C1 to C3) at room temperature often as a component of a multibed adsorbent trap. Since no single adsorbent is ideal for trapping all compounds typically found in contaminated atmospheres, it is common practice to use extraction tubes packed in series with two or more adsorbents [99,113]. Foamed polyurethanes composed of agglomerated spherical micrometer-sized particles, bonded to one another in a rigid and highly permeable structure, are suitable for sampling low-volatility organic compounds at high flow rates [112]. They are frequently used in conjunction with high-volume air samplers on account of their low pressure drop compared with standard sorbent cartridges.

18

1.4.3

Solid-Phase Extraction

High-specificity sorbents

Various sorbents based on ion exchange, bioaffinity, molecular recognition, and restricted access materials are used to achieve higher selectivity in solid-phase extraction to supplement the low-specificity sorbents discussed above [1,5,12,97]. Ion exchange is used to extract ionizable compounds and permanent ions from aqueous samples, usually, with sorbents containing immobilized ionic sites of opposite charge to the target compounds. Ions of a similar charge to the sorbent and most neutral compounds are either unretained or weakly retained and removed during the rinse step. Ion-exchange sorbents are typically classified as weak or strong depending on the identity of the immobilized ionic group and whether its charge is independent of the sample pH (strong ion exchanger) or can be manipulated by changing the sample pH (weak ion exchanger). The most common functional groups are sulfonic and carboxylic acids as strong and weak cation exchangers, respectively, and quaternary amines and secondary and tertiary amines as strong and weak anion exchangers, respectively. For many applications polymer-based and silica-based ion exchangers are interchangeable, although due to differences in nonspecific adsorption of matrix components the chemical background of the extracts may be different. Polymerbased ion-exchangers generally have a higher exchange capacity and a wider pH operating range. Disk-based solid-phase extraction is a popular method of sample cleanup for the removal of nontarget ions that interfere in the separation of the ions of interest by ion chromatography and capillary electrophoresis [117]. Mixed-mode sorbents containing cobonded ion exchange and alkyl groups in cartridge or disk format are popular for the isolation of ionizable drugs and their metabolites from biological fluids [1,12,81,118]. Target compounds are retained by a synergistic combination of ion exchange and reversed-phase interactions. The strong retention and use of efficient rinse solvents provides cleaner extracts for chromatographic analysis compared with single-mode sorbents. Surface-bonded macrocyclic ligands are used for the isolation of metal ions and some anions from aqueous samples, particularly samples of high ionic strength (e.g., sea water) or extreme pH, where ion-exchange sorbents are generally ineffective [72,119]. An enormous number of immobilized ligands with different donor atoms, ring sizes, and ligand geometry have been described. The most important are crown ethers, calixarenes, and cyclic polyamines and polysulfides. The number of donor atoms and spacer arms determine the stability of the macrocycle-metal complex while the type of donor atoms (O, S, N typically) are important for determining the metal ion selectivity. Extracted metal ions are retained in the cavity of the macrocyclic ligand until released by elution with a solution of a complexing agent with a higher binding constant for the metal. Both processes provide a high degree of specificity, allowing sorbents to be developed for single metal ions or group of metal ions. Current research is largely focused on the selective extraction of radioisotopes from nuclear decommissioning sites. Immunosorbents were used for a long time for extracting target compounds from biological systems and as immobilized reagents in assay systems based on bioaffinity. More general applications in food and environmental analysis, however, are more recent [120,121]. In part, this is due to the difficulty of raising antibodies

Core concepts and milestones in the development of solid-phase extraction

19

with a high selectivity to small molecules, as well as a lack of familiarity among analytical chemists of the procedures utilized in the production and standardization of antibodies. Fortunately, some immunosorbents are commercially available for specific compound types, such as mycotoxins, paralytic shellfish toxins, triazines, and some pesticides [121,122]. Immunosorbents are prepared by covalently bonding a suitable antibody to an appropriate sorbent. A high degree of molecular recognition is obtained based on the specificity of the antibody-antigen (target compound) interaction. High specificity allows the extraction of target compounds from complex matrices in a single step with minimal coextraction of matrix interferences. By taking advantage of cross-reactivity, class-specific immunosorbents have been developed. The development of new immunosorbents is time consuming, expensive, and requires expertise in the production, purification, and standardization of antibodies. Oligosorbents and molecularly imprinted polymers have gained traction as suitable alternatives [122]. Aptamers are short, single-stranded, synthetic oligonucleotides capable of shape recognition with a high specificity for target compounds [83,123]. After immobilization on a suitable porous particle support, they can be used as oligosorbents in solidphase extraction. They are easier to prepare than immunosorbents and afford similar molecular recognition selectivity when optimized for specific target compounds. Aptamers for a selected target compound can be screened and produced in vitro by the process known as systematic evolution of ligands by exponential enrichment followed by amplification using the polymerase chain reaction. Matrix components may affect analyte-aptamer interactions reducing the extraction efficiency. Other limitations are limited reusability and the need for specific storage conditions. Dendrimer-based sorbents are another example of synthetic extraction phases based on molecular recognition [124]. Dendrimers are macromolecules with highly repetitive branched structures. Their size, shape, inner core, and functional groups at the outer surface can be tailored to achieve molecular recognition. They are largely exploratory materials at this time. Molecularly imprinted polymers are synthetic analogs of immunosorbents that are easier and less expensive to prepare [125e128]. They are typically organic copolymers with artificially generated recognition sites able to specifically rebind target compounds in preference to other closely related compounds. The imprint is obtained by the polymerization of functional and cross-linking monomers in the presence of a template molecule and a small amount of solvent, which acts as a porogen. Originally polymers were obtained as monoliths and crushed, ground, and sieved to obtain particles of a suitable size for cartridge-based solid-phase extraction. Increasingly precipitation, multistep swelling, and emulsion polymerization are used to synthesize spherical particles of desired size today. The general approaches for synthesizing molecularly imprinted polymers have been adapted to all contemporary solid-phase extraction formats [3,23,36,53,82,83,129]. A few molecularly imprinted polymers are available commercially for high-volume applications, but otherwise, due to the specific nature of the extraction mechanism, most publications utilize experimental sorbents [129e133].

20

Solid-Phase Extraction

Choice of the template-monomer system is the key to success. In solution the template molecule complexes with one or more functional monomers, which then become spatially fixed in the solid polymer after polymerization. Methacrylic acid and 4-vinylpyridine are commonly used functional monomers for the extraction of basic and acidic compounds, respectively. Ethylene glycol dimethacrylate is widely used as the cross-linking agent, the purpose of which is to impart mechanical strength to the polymer, stabilize the molecular recognition site, and control the porosity of the polymer. The resultant imprint possesses a steric (size and shape) and chemical (spatial arrangement of complementary functional groups) memory for the template molecule. The template-functional monomer interaction needs to be reversible and if the template molecule binds too strongly to the functional monomer then sample contamination from leakage of the template molecule into the sample due to incomplete removal from the polymer can be a problem. If the functional monomer binds the template molecule too weakly, then lower specificity may result for the extraction step. Lowpolarity organic solvents are typically used in the synthesis of the imprint polymers that are usually different to the sample solvent. Shrinking or swelling of the polymers in different solvents during sample processing and recovery can have a profound effect on the extraction efficiency of the target compounds. As water is not generally a suitable solvent for synthesizing molecularly imprinted polymers, the extraction of aqueous samples can be a problem and may require a different protocol for sample processing compared with conventional approaches. Metal-organic frameworks [134,135] and covalent organic frameworks [136] are a new type of microporous crystalline materials with a uniform pore structure, high surface area, and molecular recognition capability. The molecular recognition capability can be tailored to some extent for specific applications by modifying the cavity size and functional groups available for interacting with target compounds. So far few proven applications have emerged but these experimental sorbents are considered promising for solid-phase extraction. Restricted access media were developed to facilitate the extraction of low-mass target compounds from complex biological, food, and environmental samples with minimal sample pretreatment [137e140]. They work by excluding macromolecules from the regions of the sorbent where retention occurs by adsorption, partition, or ion exchange. Exclusion from the active sampling region of the sorbent is provided by either a physical diffusion barrier, such as a pore diameter, or by a chemical diffusion barrier, such as a polymer network at the outer surface of the particle. In either case, the outer surface of the particle must be compatible and nonadsorptive of macromolecules. Restricted access media are used in all solid-phase extraction formats but the most common application is in automated online sample processing in liquid chromatography. In this case, a short precolumn packed with the restricted access sorbent is connected to the analytical column through a six-port switching valve. The sample is injected directly onto the precolumn, which retains the compounds of interest. Potentially interfering macromolecules are flushed to waste. The target compounds are then eluted from the precolumn directly onto the analytical column for separation. Simultaneously the precolumn is reconditioned (or exchanged) before processing the next sample.

Core concepts and milestones in the development of solid-phase extraction

1.4.4

21

Sorption mechanisms

A fundamental understanding of the extraction mechanism to support sorbent design and selectivity optimization requires a suitable model capable of describing the retention of target compounds under typical sample processing conditions. This quest has not proven simple as indicated by the more detailed studies of retention mechanisms in liquid chromatography [91,141]. Inorganic oxides are considered classic adsorbents with a heterogeneous surface and major contributions to retention by site-specific surface interactions with immobilized active sites [142]. Carbon is a further example of a classic adsorbent with a more homogeneous surface and significant contributions from dispersion and dipole-type interactions [105]. Silica-based chemically bonded sorbents are more complex with properties strongly affected by sample processing solvents. For predominantly aqueous samples, the major obstacles to formulating a simple extraction model are identified as the heterogeneous composition of the stationary phase, the difficulty of providing a working definition for the phase ratio, and the uncertainty as to whether the distribution mechanism for varied compounds is partition, adsorption, or some combination thereof [141,143]. For polymer films, such as poly(dimethylsiloxane) a partition mechanism from the gas phase or solution is generally assumed while contributions from interfacial adsorption is a further possibility. Solid porous polymers are assumed to act as adsorbents although solvent uptake and swelling indicate that they may also show some partition behavior [91,96]. Extraction behavior is explained by interpreting features of the extraction process in terms of an implied model. Simplicity dictates that a single model is utilized in most cases, but in practice this may be misleading, because for individual compounds different mechanisms or different contribution to a mixed mechanism is a possibility. Progress in sorbent extraction design has been made on empirical grounds with acceptable results but with only a limited understanding at a molecular level.

1.5

Theoretical contributions to modeling sample processing conditions for cartridge and disk devices

The parameters that describe the processing steps in solid-phase extraction using cartridge and disk devices are amenable to measurement by chromatographic methods or by estimation from general theoretical principles employed in chromatography with suitable modifications [13,38,144e147]. Target compound concentrations are generally low for typical sample processing conditions and a safe sampling volume, corresponding to the maximum amount of a compound that can be extracted, is determined by the breakthrough volume for the sampling device. For matrix simplification the volume and type of rinse solvent is determined by the type and amount of sorbent employed for the isolation step. To select a rinse solvent a common approach is to apply a volume of strong solvent that preserves a certain minimal retention for the least retained of the target compounds in systems using solvent desorption for recovery of extracted compounds. Maximum preconcentration of target compounds is achieved by elution with the minimal solvent volume, typically, two to three holdup volumes for

22

Solid-Phase Extraction

the sampling device. This requires a solvent yielding retention factors < 2. These requirements apply equally to thermal desorption except that larger desorption volumes can be employed if cryogenic focusing is used for band reconcentration. If conditions for recovery of the target compounds simultaneously results in some matrix components remaining immobilized on the sorbent, then additional matrix simplification is achieved at the expense of limited reuse of the sampling device. When considering a model for extraction by a cartridge-based of disk-based extraction device, it is necessary to consider that the sorption mechanism for sample application is best described by frontal analysis while rinse and desorption conditions are elution processes. Because typical solid-phase extraction devices contain short sorbent beds with a low plate number, the likelihood that sample retention is affected by kinetic properties of the sampling device cannot be ignored in practice.

1.5.1

Breakthrough volumes

The breakthrough volume is defined as the sample application volume at which a defined amount of the target compound(s) drawn through the extraction device in a continuous fashion is eluted at the exit of the extraction device [4,145e150]. To facilitate measurement of the breakthrough volume, the defined amount exiting the sampling device is typically some fraction (e.g., 1%, 5%, 10%, etc.) of the input amount and should be sufficiently large for convenient detection. The breakthrough volume, VB, is determined by its breakthrough curve, Fig. 1.8 [145]. At the start of sample application the target compounds are quantitatively retained by the sorbent up to the time the sample volume exceeds the retention capacity of the sorbent. Further sample passing through the sorbent bed is not quantitatively retained, and eventually the concentration of target compounds entering and exiting the sampling device are the same. As shown in Fig. 1.8 the retention capacity and sorption capacity of the sorbent are not the same. After breakthrough, the sampling device continues to take up the compounds of interest but inefficiently, such that an increasing amount of these compounds escapes from the sampling device in excess of that used to define the breakthrough volume. The point of inflection for the breakthrough curve corresponds to the chromatographic retention volume provided that the plate number for the sampling device is not too small. In general, there are two common causes of premature breakthrough for frontal chromatography. Sorbent overload caused by a high concentration of adsorbed target compounds or matrix components, or the sorbent bed fails to adequately retain the target compounds due to low kinetic efficiency such that the retention capacity depends on the plate number for the sampling device as well as the sample flow rate.

1.5.1.1

Experimental determination of breakthrough volumes

The simplest methods for determining breakthrough volumes are direct methods employing either online or offline detection. Online detection is especially convenient for precolumn devices used as a component of automated SPE-LC (solid-phase extraction-liquid chromatography) systems [144,145,147,149]. A solution containing

Core concepts and milestones in the development of solid-phase extraction

23

Figure 1.8 Breakthrough curve indicating the breakthrough volume, VB; the sampling volume corresponding to the saturation capacity of the sorbent, VC; and the elution volume VR for the sampling device. The standard deviation corresponding to the derivative of the curve is sV. Reproduce from Poole CF, Gunatilleka AD, Sethuraman R. Contributions of theory to method development in solid-phase extraction. J Chromatogr A 2000;885:17e39 with permission.

a low but detectable concentration of target compounds is pumped at a constant flow rate through the precolumn, which is connected directly to the liquid chromatography detector. The detector output is a breakthrough curve similar to Fig. 1.8 from which the breakthrough volume can be determined. For cartridge and disk devices, offline sample processing is typically used [96,149,151e153]. Standard solutions are processed in aliquots by the same protocol for regular samples. An offline detection method is used to determine the concentration of target compounds in the eluent from each sample aliquot. To simplify calculation each aliquot contains the same amount of target compounds but in a different sample volume. Plotting the observed recovery for the complete sampling process against the corresponding aliquot volume results in a breakthrough curve from which the breakthrough volume is estimated. For gas phase sampling, either series coupled cartridges or a serial packed bed with the same sorbent is used. With sampling employing frontal analysis conditions the breakthrough volume is determined by the presence of compounds inadequately retained on the first sorbent bed detected on the second cartridge/sorbent bed according to the direction of flow [8,9,88]. This method has the advantage that it can also be deployed in field studies to indicate breakthrough for the sampling cartridge.

1.5.1.2

Estimation methods for breakthrough volumes

The determination of breakthrough volumes is time consuming and somewhat subjective. Consequently, methods that enable breakthrough volumes to be estimated directly from compound properties or calculated from a small number of experiments are particularly attractive [38,144,147,150,154]. Cartridge-based sorbent beds only provide from 5 to 40 plates per centimeter of bed length depending on flow rate and

24

Solid-Phase Extraction

packing density [38,154]. The main contributions to the plate height are flow anisotropy, a consequence of an inadequate packing density and a relatively large particle size to achieve a low pressure drop, and resistance to mass transfer [145]. Particleloaded membranes of about 0.5 mm thickness typically provide from 5 to 9 plates with strong flow-rate dependence [38,41,155]. In both cases the conventional models for frontal analysis are not applicable and several empirical models have been developed to account for the low plate number on sample processing conditions. The Lokvist and Jonsson model is generally used for this purpose and allows the breakthrough volume to be estimated as a function of the plate number for the sampling device and a selected breakthrough level [148]. A general model for the breakthrough volume for sorbent beds with a low plate number can be written as logVB ¼ logQVM þ logð1 þ kS Þ

(1.1)

Calculation of the breakthrough volume requires measurement or knowledge of the plate number for the sampling device, N, its holdup volume, VM, and either measurement or estimation of the retention factor for the target compounds for the sample solvent. The main criteria for optimizing sample processing conditions are the dimensions of the sorbent bed (VM, N), kinetic properties (N, particle size, flow rate), and retention (N, kS, and VM) [145]. The variable Q has values  1 and is roughly constant for a defined sampling device and sampling conditions (it is estimated by the method of Lokvist and Jonsson for a specific plate number and breakthrough level). Q < 1 indicates reduced sample retention resulting from the low plate number for the sampling device at the sampling flow rate. Larger plate numbers result in a sharper front boundary and a higher breakthrough volume, and while desirable, smaller values are capable of providing useful breakthrough volumes and facilitate miniaturization of sampling devices and cost savings. For the recovery of target compounds by elution, it is necessary to consider the shape of the front as well as the retention capacity of the sorbent [144]. The required elution volume for 99% recovery, VE, on a cartridge or disk device with a low plate number can be estimated from
pffiffiffiffi VE ¼ VM ð1 þ kS Þ 1 þ 2:3 N

(1.2)

The only practical way to minimize the elution volume is to use a sorbent device with a small bed volume (minimize VM) and to use a strong solvent (kS < 3). Hendriks et al. [147] described a function based on the exponentially modified Gaussian model to accommodate peak tailing on the calculation of the elution volume.

1.5.1.3

Experimental determination of retention factors

Sorbent retention factors for sample application, rinse solvents, and elution volumes can be determined by one of three methods. The steady state equilibrium method uses a fixed volume of sample solution recycled by a pump through the sampling device and back to the sample reservoir until a steady state is obtained [150]. The target

Core concepts and milestones in the development of solid-phase extraction

25

compounds retained by the sorbent are then recovered by elution and quantified. The retention factors are calculated from the volume and concentration of the sample solution, the holdup volume for the sorbent bed, and the amount of each target compound taken up by the sorbent under steady state conditions assuming a linear sorption isotherm. The shake-vial equilibrium method has lower accuracy but is easier to perform [146,156]. A solution with a known concentration of the target compounds is shaken with a known amount of sorbent until equilibrium is reached. Sampling the solution phase at equilibrium allows the adsorbed amount of each compound to be calculated. The retention factors are then calculated from the solid-liquid distribution constants and the phase ratio for the sampling device (estimated in a separate experiment). The direct measurement of sorbent retention factors by liquid chromatography in columns packed with sorbent is straightforward and uses the same protocols employed in column characterization [147,153,154,157]. Forced-flow planar chromatography can be used to determine retention factors for particle-loaded membranes and particle-embedded membranes [41,151,155]. Compounds with retention factors up to about 10,000 can be determined by the direct method, which is a more than adequate range for sampling purposes, as compounds with larger values will be more than adequately retained and only need be approximated.

1.5.1.4

Estimation of retention factors

The measurement of retention factors takes additional time and is not possible at all if standards are unavailable. Methods that allow estimation of retention factors from structure and sorbent retention properties for different conditions are useful for the initial phase of sorbent selection and to identify initial trial conditions for sample processing. For aqueous samples or samples in the gas phase the solvation parameter model is easily adapted to this problem [145,153,154]. For aqueous solutions log kS ¼ c þ eE þ sS þ aA þ bB þ vV

(1.3)

and for sampling from the gas phase log kS ¼ c þ eE þ sS þ aA þ bB þ lL

(1.4)

The model equations contain product terms representing target compound properties (descriptors), indicated by capital letters, and the complementary system properties (sorbent and sampling device), indicated by lowercase letters [13,143,159e161]. Each product term defines the relative contribution of a specific intermolecular interaction to the correlated property, in this case log kS. The contribution from electron lone pair interactions (or the additional dispersion forces that result from the larger polarizability of compounds with weakly held electrons) is defined by eE, interactions of a dipole-type (orientation and induction) sS, hydrogen-bonding interactions by aA (sorbent hydrogen-bond basicity and compound hydrogen-bond acidity) and bB (sorbent hydrogen-bond acidity and compound hydrogen-bond basicity), and the difference in cavity formation and dispersion interactions for transfer of a compound between

26

Solid-Phase Extraction

condensed phases (vV) or from the gas phase to a condensed phase (lL). The compound descriptors are formally defined as excess molar refraction E, dipolarity/polarizability S, effective hydrogen-bond acidity A, effective hydrogen-bond basicity B, McGowan’s characteristic volume V, and the gas-liquid partition constant on hexadecane at 25 C, L [162e164]. The compound descriptors are not discussed further here except to note that values for thousands of compounds are available from the literature; methods for determining their values from solubility, chromatographic retention factors, and liquid-liquid distribution constants are available; and software products (fragmentation methods) for their estimation from structure are also available [162e164]. The constant in Eqs. (1.3) and (1.4), c term, is not a compound property but is required to estimate retention factors. It is a complex function of factors of which the most important is the phase ratio of the sampling system. Consequently, model equations are specified for a specific sampling device while the system constants (e, s, a, b, v, l) are characteristic of sorbent properties. The solvation parameter model was devised from a cavity model of solvation based on partition. It is generally applicable to adsorption at organic surfaces with flexible bound groups and polymer surfaces but is unsuitable for describing adsorption on inorganic oxides characterized by sitespecific and size-dependent interactions. The model also applies to neutral compounds, and while ionizable compounds and ions can be handled in different ways, this is not a typical application of the model in solid-phase extraction [143,159]. Ionization tends to reduce retention compared with neutral compounds except for ion-exchange sorbents. Ionization suppression using buffers is often a practical solution to increasing retention for ionizable compounds on neutral sorbents. System maps are typically used for selection or evaluation of sampling system for a chosen application [13,143,145]. A system map is a plot of the system constants of the solvation parameter model as a function of solvent composition (binary or ternary solvents) or temperature (for gas chromatography). A system map for the sorbent Oasis HLB is shown in Fig. 1.9 for methanol-water compositions from 0% to 50% (v/v) methanol [158]. Each system constant changes smoothly with the independent variable and can be fitted to simple linear or second order polynomial functions for computeraided simulation of sampling conditions. The left-hand side of the map, corresponding to low organic solvent, is the region of interest for establishing a safe sampling volume using Eq. (1.3) to estimate kS and Eq. (1.1) to compute the breakthrough volume. The intermediate region of the system map is of interest for the selection of rinse solvents using Eqs. (1.2) and (1.3) to estimate a suitable solvent strength to elute matrix components while providing sufficient retention to immobilize the weakest retained of the target compounds. The right-hand side of the map is used to identify a strong solvent to elute the most retained of the target compounds with kS < 3 and preferably kS z 0. The selection of low-specificity sorbents for a particular application is based on identifying an appropriate sorbent chemistry for the extraction that provides the desired breakthrough volume for the target compounds [145,153,158]. Only system constants with a positive sign contribute to sorbent retention. For Oasis HLB, this corresponds to the compound size (v system constant) and electron lone pair interactions (e system constant). Interactions of a dipole type (s system constant) and sorbent hydrogen-bond basicity (a system constant) are close to zero and of minor importance for explaining

Core concepts and milestones in the development of solid-phase extraction

27

Figure 1.9 System map for OASIS HLB with methanol-water solvent compositions.

retention on Oasis HLB. The hydrogen-bond acidity (b system constant) is large and negative signifying that this parameter is influential in the retention of compounds of roughly the same size but with different hydrogen-bond basicity. Fig. 1.9 is fairly typical of results for aqueous samples in which the retention mechanism is dominated by the characteristic properties of water: its strong cohesion favoring transfer to the sorbent (v system constant) and strong hydrogen-bond acidity reducing the retention of hydrogen-bond basic compounds (b system constant). Individual sorbents typically vary in the relative contribution of the system constants to extraction but no sorbent is able to decouple the dominant influence of the characteristic properties of water on extraction for either low-specificity sorbents or more selective polar sorbents. For samples soluble in organic solvents specific sorbent interactions are usually competitive with solute-solvent interactions and a wider range of retention properties are observed. However, overall retention is often weaker than for aqueous samples because the cohesion of the sample solution and solvated sorbent are of a similar value and the major driving force observed for aqueous extraction is now missing and replaced by individual polar interactions, which are usually weaker in comparison.

1.6

Conclusions

Solid-phase extraction is continuing to develop in response to changing laboratory needs driven by the desire for increased automation, miniaturization, and adoption of green chemical principles. This is reflected in new devices and sorbent chemistry which are active research fields today. While chromatographic sorbents have evolved into specialized and complex formats the synthesis of solid-phase extraction sorbents is less demanding and presents a lower barrier to the development and application of new chemistries for sample preparation. Thus, a large portion of materials that attract the most interest in the contemporary literature are experimental materials in contrast to the large number of purely application papers that employ commercially available materials. This has produced a gap in capabilities between a stable commercial sector

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Solid-Phase Extraction

supporting established formats of solid-phase extraction and a research dominated sector continuing to invent and develop new possibilities, some of which show potential to cross-over into the commercial sector. These are interesting times as sample preparation is recognized as the bottleneck to higher throughput in instrumental analysis and is gaining increasing attention as the problem to solve in the 21st century. It is also clear that the likely solution(s) will have to be innovative and involve novel technologies.

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[99] Matisova E, Skrabkova S. Carbon sorbents and their utilization for the preconcentration of organic pollutants in environmental samples. J Chromatogr A 1995;707:145e79. [100] Hennion M-C. Graphitized carbons for solid-phase extraction. J Chromatogr A 2000;885: 73e95. [101] de Toffoli AL, Maciel EVS, Fumes BH, Lancas FM. The role of graphene-based sorbents in modern sample preparation techniques. J Sep Sci 2018;41:288e302. [102] Ravelo-Perez LM, Herrera-Herrera AV, Hernandez-Borges J, Rodriguez-Delgado M-A. Carbon nanotubes: solid-phase extraction. J Chromatogr A 2010;1217:2618e41. [103] Liang X, Liu S, Wang S, Guo Y, Jiang S. Carbon-based sorbents: carbon nanotubes. J Chromatogr A 2014;1357:53e67. [104] Socas-Rodriguez B, Herrera-Herrera AV, Asensio-Ramos M, Hernandez-Borges J. Recent applications of carbon nanotube sorbents in analytical chemistry. J Chromatogr A 2014;1357:110e46. [105] Lepont C, Gunatilleka AD, Poole CF. Retention characteristics of porous graphitic carbon in reversed-phase liquid chromatography with methanol-water mobile phases. Analyst 2001;126:1318e25. [106] Boonjob W, Miro M, Segundo MA, Cerda V. Flow-through dispersed carbon nanofiberbased microsolid-phase extraction coupled to liquid chromatography for automatic determination of trace levels of priority environmental pollutants. Anal Chem 2011;83: 5237e44. [107] Zhang B-T, Zheng X, Li H-F, Lin J-M. Application of carbon-based nanomaterials in sample preparation. Anal Chim Acta 2013;784:1e17. [108] Seethapathy S, Gorecki T. Applications of polydimethylsiloxane in analytical chemistry: a review. Anal Chim Acta 2012;750:48e62. [109] Gilart N, Marcee RM, Borrull F, Fontanals N. New coatings for stir-bar sorptive extraction of polar emerging organic contaminants. Trends Anal Chem 2014;54:11e23. [110] Che D, Chang J, Ji Z, Zhang S, Li G, Sun Z, You J. Recent advances and applications of polydopamine-derived adsorbents for sample pretreatment. Trends Anal Chem 2017;97: 1e14. [111] Szultka-Mlynska M, Olszowy P, Buszewski B. Nanoporous conducting polymer-based coatings in microextraction techniques for environmental and biomedical applications. Crit Revs Anal Chem 2016;46:236e7. [112] Camel V, Caude M. Trace enrichment methods for the determination of organic pollutants in ambient air. J Chromatogr A 1995;710:3e19. [113] Ramirez N, Cuadras A, Rovira E, Borrull F, Marce RM. Comparative study of solvent extraction and thermal desorption methods for determining volatile organic compounds in ambient air. Talanta 2010;82:719e27. [114] Helmig D, Vierling L. Water-adsorption capacity of the solid adsorbents Tenax-TA, Tenax eGR, Carbotrap, Carbotrap-C, Carbosieve-SIII, and Carboxen-569 and water management-techniques for the atmospheric sampling of volatile organic trace gases. Anal Chem 1995;67:4380e6. [115] Bradley RH. Recent developments in the physical adsorption of toxic organic vapors by activated carbons. Adsorp Sci Technol 2011;29:1e28. [116] Foley C. Carbogenic molecular-sieves e synthesis, properties and applications. Microporous Mater 1995;4:407e33. [117] Fritz JS, Gjerde DT. Ion chromatography. Weinheim: Wiley-VCH; 2009. [118] Fontanals N, Cormack PAG, Marce RM, Borrull F. Mixed-mode ion-exchange polymeric sorbents: dual-phase materials that improve selectivity and capacity. Trends Anal Chem 2010;29:765e79.

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Solid-Phase Extraction

[119] Rahman IMM, Begum ZA, Hasegawa H. Selective separation of elements from complex solution matrix with molecular recognition plus macrocycles attached to a solid-phase: a review. Microchem J 2013;110:485e93. [120] Hennion M-C, Pichon V. Immuno-based sample preparation for trace analysis. J Chromatogr A 2003;1000:29e52. [121] Senyuva HZ, Gilbert J. Immunoaffinity column clean-up techniques in food analysis: a review. J Chromatogr B 2010;878:115e32. [122] Pichon V, Combes A. Selective tools for the solid-phase extraction of Ochratoxin A from various complex samples: immunosorbents, oligosorbents, and molecularly imprinted polymers. Anal Bioanal Chem 2016;408:6983e99. [123] Pichon V, Brothier F, Combes A. Aptamer-based-sorbents for sample treatment e a review. Anal Bioanal Chem 2015;407:681e98. [124] Sajid M. Dendimers based sorbents: promising materials for analytical extractions. Trends Anal Chem 2018;98:114e27. [125] Haginaka J. Molecularly imprinted polymers as affinity-based separation media for sample preparation. J Sep Sci 2009;32:1548e65. [126] Beltran A, Borrull F, Cormack PAG, Marce RM. Molecularly imprinted polymers: useful sorbents for selective extraction. Trends Anal Chem 2010;29:1363e75. [127] Turiel E, Martin-Esteban A. Molecularly imprinted polymers for sample preparation: a review. Anal Chim Acta 2010;688:87e99. [128] Alvarez-Lorenzo C, Concheiro A, editors. Handbook of molecularly imprinted polymers. Shrewsbury: Smithers Rapara Technology; 2013. [129] Huang SY, Xu JQ, Zheng JT, Zhu F, Xie LJ, Ouyang GF. Synthesis and applications of magnetic molecular imprinted polymers in sample preparation. Anal Bioanal Chem 2018; 410:3991e4014. [130] Li GZ, Row KH. Recent applications of molecularly imprinted polymers (MIPs) on micrextraction techniques. Sep Purif Revs 2018;47:1e18. [131] Speltini A, Scalabrini A, Marash F, Stierini M, Profumo A. Newest applications of MIPs for extraction of contaminants from environmental and food matrices: a review. Anal Chim Acta 2017;974:1e26. [132] Ansari S, Karimi M. Novel developments and trends of analytical methods for drug analysis in biological and environmental samples by molecularly imprinted polymers. Trends Anal Chem 2017;89:146e62. [133] Martin-Esterban A. Recent MIP-based sample preparation techniques in environmental analysis. Trends Env Anal Chem 2016;9:8e14. [134] Wang YH, Rui M, Lu GH. Recent applications of metal-organic frameworks in sample pretreatment. J Sep Sci 2018;41:180e94. [135] Hashemi B, Zohrabi P, Raza N, Kim KH. Metal-organic frameworks as advanced sorbents for the extraction and determination of pollutants from environmental, biological and food media. Trends Anal Chem 2017;97:65e82. [136] Qian H-L, Yang C-X, Wang W-L, Yang C, Yan X-P. Advances in covalent organic frameworks in separation science. J Chromatogr A 2018;1542:1e18. [137] de Faria HD, de Carvalho Abrao LC, Santos MG, Barbosa AF, Figueiredo EC. New advances in restricted access matrials for sample preparation: a review. Anal Chim Acta 2017;959:43e65. [138] Souverain S, Rudaz S, Veuthey JL. Restricted access materials and large particle supports for on-line sample preparation: an attractive approach for biological fluid analysis. J Chromatogr B 2004;801:141e56.

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[139] Yang SH, Fan H, Classon RJ, Schug KA. Restricted access media as a streamlined approach towards on-line sample preparation: recent advancements and applications. J Sep Sci 2013;36:2922e38. [140] Popov A, Blinnikova ZK, Tsyurupa P, Davankov VA. Hypercrosslinked polymeric restricted access materials for the analysis of biological fluids. J Sep Sci 2018;41:3302e9. [141] Poole CF. Influence of solvent effects on the retention of small molecules in reversedphase liquid chromatography. Chromatographia 2019;82:49e64. [142] Kiridena W, Poole CF. Influence of solute size and site-specific surface interactions on the prediction of retention in liquid chromatography using the solvation parameter model. Analyst 1998;123:1265e70. [143] Poole CF. Chromatographic test methods for characterizing alkylsiloxane-bonded silica columns for reversed-phase liquid chromatography. J Chromatogr B 2018;1092:207e19. [144] Hennion M-C. Solid-phase extraction: method development, sorbents, and coupling with liquid chromatography. J Chromatogr A 1999;856:3e54. [145] Poole CF, Gunatilleka AD, Sethuraman R. Contributions of theory to method development in solid-phase extraction. J Chromatogr A 2000;885:17e39. [146] Ortega L, Lopez R, Cacho J, Ferreira V. Use of solid-liquid distribution coefficients to determine retention properties of Porapak-Q resins. Determination of optimal conditions and beta-damascenone from wine. J Chromatogr A 2001;931:31e9. [147] Hendriks G, Uges DRA, Franke JP. New practical algorithm for modeling analyte recovery in bioanalytical reversed phase and mixed-mode solid-phase extraction. J Pharm Biomed Anal 2008;48:158e70. [148] Lokvist P, Jonsson J-A. Capacity of sampling and preconcentration columns with a low number of theoretical plates. Anal Chem 1987;59:818e21. [149] Bielicka-Daszkiewicz K, Voelkel A. Theoretical and experimental methods of determination of the breakthrough volume of solid-phase extraction sorbents. Talanta 2009;80: 614e21. [150] Gelencser A, Kiss G, Krivacsy Z, Varga-Puchony Z, Hlavay J. A simple method for the determination of capacity factor on solid-phase extraction cartridges. I. J Chromatogr A 1995;693:217e25. [151] Larrivee ML, Poole CF. A solvation parameter model for the prediction of breakthrough volumes in solid-phase extraction with particle-loaded membranes. Anal Chem 1994;66: 139e46. [152] Mayer ML, Poole CF. Identification of the procedural steps that affect recovery of semivolatile compounds by solid-phase extraction using cartridge and particle-loaded membrane (disk) devices. Anal Chim Acta 1994;294:113e26. [153] Seibert DS, Poole CF. A general model for the optimization of sample processing conditions by solid-phase extraction applied to the isolation of estrogens from urine. J High Resolut Chromatogr 1998;21:481e90. [154] Miller KG, Poole CF. Methodological approach for evaluating operational parameters and the characterization of a popular sorbent for solid-phase extraction by high pressure liquid chromatography. J High Resolut Chromatogr 1994;17:125e34. [155] Fernando WPN, Larrivee ML, Poole CF. Investigation of the kinetic properties of particle-loaded membranes for solid-phase extraction by forced flow planar chromatography. Anal Chem 1993;65:588e95. [156] Ferreira V, Jarauta I, Ortega L, Cacho J. Simple strategy for the optimization of solidphase extraction procedures through the use of solid-liquid distribution coefficients. Application to the determination of aliphatic lactones in wine. J Chromatogr A 2004; 1025:147e56.

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Solid-Phase Extraction

[157] Poole CF, Poole SK. Foundations of retention in partition chromatography. J Chromatogr A 2009;1216:1530e50. [158] Dias NC, Poole CF. Mechanistic study of the sorption properties of OASIS HLB and its use in solid-phase extraction. Chromatographia 2002;56:269e75. [159] Poole CF, Poole SK. Column selectivity from the perspective of the solvation parameter model. J Chromatogr A 2002;965:263e99. [160] Vitha MF, Carr PW. The chemical interpretation and practice of linear solvation energy relationships in chromatography. J Chromatogr A 2006;1126:143e94. [161] Abraham MH, Poole CF, Poole SK. Classification of stationary phases and other materials by gas chromatography. J Chromatogr A 1999;842:79e114. [162] Abraham MH, Ibrahim A, Zissimos AM. Determination of sets of solute descriptors from chromatographic measurements. J Chromatogr A 2004;1037:29e47. [163] Poole CF, Atapattu SN, Poole SK, Bell AN. Determination of solute descriptors by chromatographic methods. Anal Chim Acta 2009;652:32e53. [164] Poole CF, Ariyasena TC, Lenca N. Estimation of the environmental properties of compounds from chromatographic measurements and the solvation parameter model. J Chromatogr A 2013;1317:85e104.

Inorganic oxide and chemically bonded sorbents

2


ska 1 , Hossam Al-Suod 1,2 , Bogusław Buszewski 1, 2 Małgorzata Szultka-Młyn 1 Department of Environmental Chemistry and Bioanalytics, Faculty of Chemistry, Nicolaus Copernicus University, Torun, Poland; 2Centre for Modern Interdisciplinary Technologies, Nicolaus Copernicus University, Torun, Poland

2.1

Introduction

Sample preparation is a crucial component of modern environmental, biomedical, and pharmaceutical analysis and often involves elaborate, time-consuming procedures (app. 80% of the total analysis time). Choosing an appropriate sample preparation method is most important in the qualitative and quantitative determination of target compounds. A large number of target compounds are of interest, ranging from highly lipophilic to moderately polar and exhibiting basic, acidic, or neutral properties. In addition, this step is required for several reasons: to eliminate a possible compound interference, to concentrate and stabilize the target compounds, and to achieve the conditions suitable for the final chromatographic or other analysis [1e3]. The most common sample preparation method is solid-phase extraction (SPE). Numerous SPE applications have been developed and are summarized in several reviews [1,4e11]. Generally, during the extraction process, an aqueous sample passes over a solid phase with the transfer of the target compounds to the sorbent from which they are subsequently recovered by elution with a suitable solvent. A successful SPE is based on a suitable and selective distribution of analytes to the solid phase from the sample solvent described by a distribution constant for low analyte concentrations for which a linear sorption isotherm might be assumed. The extraction requires that the target compounds have a greater affinity for the solid phase than for the sample matrix [12]. Insufficient retention by the solid phase results in low breakthrough volumes. An important strategy to overcome inadequate breakthrough volumes is to optimize the sorbent type [13e16]. Due to good reproducibility, precision, relatively low cost, operational simplicity, and the possibility of combining the SPE technique directly (online or off-line) with other methods, has made it one of the most widely used techniques for the routine analysis of pharmaceutical, cosmetic, biomedical, and environmental samples, in controlling technological processes, and for foods and fodder. Moreover, it is used for selective isolation and enrichment of analytes from gaseous, liquid, and solid matrices.

Solid-Phase Extraction. https://doi.org/10.1016/B978-0-12-816906-3.00002-9 Copyright © 2020 Elsevier Inc. All rights reserved.

38

2.2

Solid-Phase Extraction

Inorganic oxide sorbents

The selection of an appropriate sorbent material is a critical point in SPE since it ensures suitable retention of the analytes and their easy release for high recovery upon elution. Nowadays, a variety of sorbents are available for SPE, which can be divided into three general categories: inorganic oxides, low-specific sorbents, and compound-specific/class-specific sorbents. Silica, alumina (Al2O3), and Florisil (a synthetic hydrated magnesium silicate) are examples of typical inorganic oxide sorbents. Their large surface area and high activity increase the retention based on the type, number, and location of the complementary analyte functional groups. Nonpolar compounds, such as aromatic and alkenes with polarizable functional groups, are poorly retained by these sorbents, while polar compounds with hydrogen bonding functional groups are highly retained (e.g., sulfonic acid, carboxylic acid, phenol, etc). Typical silica gels used for SPE have surface areas of 300e800 m2/g, a pore size of 4e10 nm, and an apparent pH of 5.5e7.5. Alumina typically used for SPE has a surface area of about 150 m2/g, a pore size of 6 nm, and is available in neutral, weakly acidic, acidic, and basic forms depending on the processing conditions. Florisil has a surface area of about 250e300 m2/g and an apparent pH of about 8.5. Limited pH stability, especially for samples with a pH > 8, is the most important weakness of silica gel. Other metal oxides, such as zirconia (ZrO2) and titania (TiO2), have significantly better pH stability, but their surface chemistry is more complex, and they are not as popular as silica for SPE applications. The solution to the limited pH stability of silica is the synthesis of organic-inorganic hybrid sorbents stable up to pH 12. Common applications of inorganic oxide sorbents include the isolation of compounds such as pesticides or polycyclic aromatic hydrocarbons (PAHs) from aqueous samples [17,18], polychlorinated biphenyls (PCBs) and organochlorine pesticides [19], as well as mycotoxins in herbal matrices [20]. Inorganic oxide sorbents were used for the isolation of N-acyl homoserine lactones from soil [21].

2.2.1

Physicochemical properties

The term silica denotes a substance with a stoichiometric formula SiO2, as well as its hydrated form SiO2
H2O. Proper classification of silica requires consideration of four basic parameters: (1) crystal structure; (2) dispersion (solid silica dispersed in gaseous or liquid dispersion medium), silica hydrogels (commonly called silica gels); (3) porosity; and (4) structural and surface heterogeneity [22e25]. Furthermore, there also exist soluble silica, silica sols, and solutions of polymerized silica built up of branched siloxane chains [13,14,22e24]. According to Tanaka [26], the pores in wide-pore materials differ significantly within the same sample. Pore width is a median value calculated on the basis of the pore size distribution. Three basic pore size distribution curves can be distinguished: homogenous, with a size distribution close to a bell-shaped curve; bimodal, with two maxima; and heterogenous, with several peaks corresponding to the populations of pores of different sizes. Pore size is reflected in the specific surface area of the adsorbent. The external surface area Se corresponds to the surface for porous spherical

Inorganic oxide and chemically bonded sorbents

39

particles per gram of silica. It is inversely proportional to the bead size dp and density r. In turn, the internal surface area Si consists of the surfaces of the internal pore walls, with the exception of closed pores. A large specific surface area (the sum of its external and internal surface) is indicative of the presence of pores of a small diameter. In turn, a low specific surface area indicates the presence of a population of macropores [25]. In addition to shape and pore size, another factor characterizing sorbent porosity is the specific volume (Vp) described as the amount of liquid that will fill the pores completely, calculated per gram of silica [27]. By analogy to the specific surface area, the total pore volume depends on pore size. According to the postulate of Brunauer, Emmet, and Teller (the BET method) [28e30], sorption isotherms of different shapes result from different mechanisms for the adsorption and desorption of vapors from pores of a different size. Sorption properties of porous silica are the result of the existence of globular or sponge-like matrix structure [31e33]. Optimal adsorbent for SPE is characterized by good mechanical properties and a limited number of micropores that adversely affect the mass exchange and diffusion [33e35], but also by a uniform pore shape [22e25]. A silica sample of suitable morphology is illustrated in Fig. 2.1. Silica gel is the base material commonly used for the synthesis of chemically bonded phases with complementary properties to native silica gel for use in SPE. It is relatively easy to synthesize in commercial quantities with uniform characteristics at a low cost [23,24].

Figure 2.1 Scanning electron microsope images of a silica adsorbent with a bead diameter of 35 mm (scale 1:1500).

40

Solid-Phase Extraction

The surface heterogeneity of silica gel is a result of various functional groups e different types of silanols and siloxanes. Structural heterogeneity results from the differences in pore shapes (cylindrical, funnel-shaped, and ink-bottle shaped) associated with the three-dimensional skeleton of SiO2. Silica used for SPE extraction is essentially porous and noncrystalline with an abundance of surface silanol groups. Silica gel is a useful adsorbent, and on account of its rigid skeleton does not swell or shrink in common solvents and has good mechanical strength [13,14]. The silanol groups are responsible for the polar adsorption centers. Three types of silanol groups can be distinguished: isolated (free, single); vicinal or bridged (total content 60%e65%); and geminal (10%e12%) [36e38]. The relative concentration and type of silanol groups depend on the hydrothermal or heat treatment of the silica particles. Some typical physicochemical properties of silica sorbents are summarized in Table 2.1. Table 2.1 Physicochemical characterization of silica sorbents [38].

Symbol [units]

“Typical”

Criteria “Theoretically optimal”

Specific surface area

SBET [m2g1]

150e500

320

Mean pore diameter

D [nm]

10e30

10

Mean particle size

dp [mm]

20e60

40

Laser counter microscopy

Interparticle column porosity

ε [-]

0.4

0.84

Chromatography

Trace amounts of metals

-[ppm]

e

500

Inductively coupled plasma (spectroscopy), atomic absorption spectroscopy

Powder density

ss [g cm1]

0.4e0.6

0.45

Pycnometry

Particle shape

e

Spherical

Strictly spheroidal

Microscopy

Surface pH

e

e

2pH9

pH-metry of dispersions

Concentration of OH groups

aOH [mmol m2]

8

5 aOH8

Spectrometry, chemical methods

Characteristic value

Measurement technique Low-temperature adsorptiondesorption of nitrogen or helium

Inorganic oxide and chemically bonded sorbents

2.2.2

41

Silica-based chemically bonded sorbents

Silica can be used as an SPE sorbent without further modification of silanols. However, in order to increase its applicability and selectivity, the silica surface is usually modified by bonding different functional groups to the surface. Silica gel may be modified to obtain chemically bonded sorbents with hydrophobic or hydrophilic functional groups. These modifications may be carried out using different methodologies. Chemically bonded sorbents can be grouped into several categories based on the chemical nature of the embedded functional groups, the nature of the support, as well as their structure. Based on structure, Unger suggests a division into several categories [36]. The first group includes sorbents obtained by the action of a single modifier (monolayer structure). The multilayer may be bonded but may also be coated onto the silica surface. A second type is chemically bonded sorbents with a diffusion barrier, which protect bonded ligands. Mixed-mode sorbents have various types of functional groups affording multiple interaction mechanisms that facilitate their use in a wider range of applications. Lastly, another category of chemically bonded sorbents has a “sandwich” structure resulting from polymerization. Generally, the preparation of amorphous silica gel is carried out with the benign and convenient solegel methodology. This type of polymerization involves two chemical steps, namely hydrolysis and condensation, which results in the formation of SieOeSi chemical bonds from a silicic acid monomer, such as sodium silicate, or silicic acid esters (e.g., tetraethylorthosilicate, TEOS). The monomers are polymerized to a specific molecular weight or size. The initial form of the prepolymerized silica is a sol, which is further agglomerated to the shape of the final products. The silanol groups present on the silica gel surface result in the formation of siloxane bonds by reaction with silane reagents. Depending on the type of silane, the chemically bonded sorbent surface can be monomeric (Fig. 2.2A) or polymeric (Fig. 2.2B). The synthesis procedure may be divided into two stages (Fig. 2.2C): (a) silanization and (b) hydrosilylation. For the reaction of surface silanol groups to form siloxane bonds, chloro-, amino- or methoxy/ethoxy- silanes are used [9,23,31,37e39]. The ratio of the amount of silane reagent to silica and the number and type of silanol groups provides the possibility to obtain sorbents with controlled coverage density [24]. Alternatively, a stable bond between silicon and carbon (SieC) can be formed while the attachment of the organic moiety takes place. For this purpose, alkynes, cyano, and other ligands are used. The reaction between silicon hydrides and alkenes is the preferred laboratory method to form SieC bonds [23,24,37,38]. Luo et al. [40] successfully prepared humic acid-bonded silica via an amide linkage between humyl chloride and the amino group of APTS-bonded silica as a novel sorbent for the solid-phase extraction of benzo[a]pyrene in edible oils, Fig. 2.3. A high recovery of benzo[a]pyrene ranging between 93.3% and 100.1% was obtained. In addition, Petrova [41] prepared two new sorbents, silica gel modified with Cystine (Sig-Cys-S-S-Cys) and silica gel modified with N-Benzyloxycarbonyl-L-Methionine (Sig-Z-Met-OH) and evaluated them for the quantitative extraction of Au(III) from hydrochloric acid solutions. Under optimized conditions, less than 43% of Au(III)

(A) R' Si

OH

Si

R′ 2

+

X

R

Si R'

R

Si

+

2 HX

R'

R'

Si

OH

Si

O

O

Si

R

R'

(B) OH Si

OH X

O Si

+

OH

O Si

2

Si

X

R

O

Si

Si

OH

Si

O Si

R O

+

2 HX

O

X

OH

Si O

R

OH

(C) O O Si OH + (EtO)3SiH O

O

O

Si O

Si

O

O

H + 3 EtOH

(a)

(b)

O O Pt-catalyst O Si CH2CH2R O Si H + H2C = CH– R – Toluene O O

Figure 2.2 Synthesis of (A) monomeric and (B) polymeric chemically bonded sorbents and (C) silica hydride. H 2N

Si(OCH3)3

3-Aminopropyltrimethoxysilane (APTS) (humic acid) R-COOH Silica R Reflux O SOCl2 H2N H 2N 48h HN CH3O

R HN

O

Si R-COCl CH3O OCH3 Si OCH3 Si OCH3 Si OCH3 CH3O O CH O 3 O DMF, Triethylamine O O

Figure 2.3 The scheme to prepare humic acid-bonded silica as an SPE adsorbent.

Inorganic oxide and chemically bonded sorbents

43

was recovered, whereas Sig-Z-Met-OH enabled fast and quantitative extraction of Au(III) from 0.1 to 0.01 mol1 HCl.

2.3

Evaluation of formats, sorbent types, and modes of interaction

SPE sorbents are available in several formats: contained within cartridges, in columns fashioned like syringe barrels, or in disks, Fig. 2.4. Typical columns are manufactured of polypropylene or glass, and the sorbent is held within the column by porous frits made of polyethylene or polytetrafluoroethylene (PTFE) [12]. A typical format is a syringe barrel consisting of a 20 mm frit at the bottom of the barrel, with the relevant sorbent above and an additional frit at the top. Alternatively, disks of various sizes with sorbent particles homogeneously distributed in a support matrix with a diameter generally significantly larger than its height, pipette tips, and 96-well SPE microtiter plates (which use the disk format). There are two common types of disk formats, disc, particle-loaded membranes (PLMs), which are composed of polytetrafluoroethylene microfibrils and enclosed sorbent particles with a diameter of about 8 mm, and particle-embedded glass fiber discs (PEGFDs) [37]. The disk format is advantageous for handling small sample sizes and for sampling at relatively high flow rates with disks of high cross-sectional area. Last, a small column connected online to a chromatographic system facilitating automated sample handling and separation. The 96-well plate format also facilitates automation and high-throughput sample preparation using liquid handling devices and parallel sample processing. Native silica is typically used for the extraction of nonpolar and low-polarity analytes dissolved in organic solvents. Chemically bonded sorbents (C1, C2, C8, C18, Ph, CN, NH2) can be used for similar samples but are more commonly used for the extraction of neutral compounds of wide polarity from aqueous samples. The effectiveness of SPE methods depends on several factors, such as the sorbent chemistry, pH (ionizable compounds), sample pretreatment, organic solvents used for washing and eluting steps, as well as the flow rate during the different steps. The elution of the analytes is effected by a suitable solvent, leaving the interfering substances on the column. Strong and weak elution solvents for adsorbed compounds are described in Table 2.2. To achieve selectivity SPE provides countless diverse choices of sorbents, ranging from the traditional reversed-phase sorbents (C18, C8), normal phase (silica, alumina), ion exchange, mixed-mode (ion exchange and reversed phase) and functionalized resins based on styrene-divinylbenzene (SDVB) polymers [1,5,38]. The interactions between the target compounds and the sorption centers on the solid phase include hydrophobic interactions and hydrophilic interactions such as dipole-dipole, induced dipole-dipole, hydrogen bonding, and pp interactions. In addition, for analytes with charged functional groups, electrostatic interactions contribute to selectivity on sorbents with complementary charged functional groups. Affinity-type interactions

44 Solid-Phase Extraction

Figure 2.4 Different formats for SPE: (A) tube, (B) Speedisk, (C) fiber, (D) chip and (E) magnetic silica-based core/shell nanoparticles.

Type of bed sorbent

Structure of bound ligand

Analyte type

Dissolving solvents

Elution solvents

Slightly-moderately nonpolar-nonpolar

Methanol/water; Acetonitrile/water

Hexane, chloroform (nonpolar analytes); Methanol (polar analytes)

Slightly-moderately polarstrongly polar

Hexane, chloroform, acetone

Methanol

Slightly-moderately polarstrongly polar

Hexane, chloroform

Methanol (dependent on the type of analyte)

Reversed phases Octadecyl (C18)

-(CH2)17CH3

Octadecyl (C18) LightLoad

-(CH2)17CH3

Octadecyl (C18) PolarPlus

-(CH2)17CH3

Octyl (C8)

-(CH2)7CH3

Ethyl (C2)

-CH2CH3

Cyclohexyl

-CH2CH2-

Phenyl

-CH2CH2CH2-

Inorganic oxide and chemically bonded sorbents

Table 2.2 Sorbents applied in SPE.

Normal phases (bonded) Cyano (CN)

-(CH2)3CH

Amino (NH2)

-(CH2)NH2

Diol (COHCOH)

-(CH2)3OCH2CHCH2OHOH

Normal phases (adsorption) Silica gel

-SiOH

Florisil

Mg2SiO3

Alumina

Al2O3 45

Continued

Type of bed sorbent

Structure of bound ligand

46

Table 2.2 Sorbents applied in SPE.dcont’d Analyte type

Dissolving solvents

Elution solvents

Anion exchange-ionic acid Cation exchange-ionic base

Water or buffer (pH¼pKaþ2) Water or buffer (pH¼pKa-2)

a) buffer (pH¼pKaþ2) b) pH where sorbent or analyte is neutral c) solvent with high ionic strength a) buffer (pH¼pKa-2) b) pH where sorbent or analyte is neutral c) solvent with ionic strength

Dextran

-

-

-

RP butyl (C4)

-(CH2)3CH3

-

-

-

HI HI-propyl (C3)

-(CH2)2CH3

IE CBX (carboxylic acid)

-COOH

PEI (polyethyleneimine)

-(CH2CH2NH)n-

Ion exchangers (anion and cation exchange) Amino (NH2) o

o

-(CH2)3NH2

1 , 2 -Amino (NH/ NH2)

-(CH2)3NHCH2CH2NH2

Quaternary amine (Nþ)

-(CH2)2COOH

Carboxylic acid (COOH)

-(CH2)3Nþ(CH3)2

Acid (SO2OH)

-(CH2)3SO2OH

Acid (ArSO2OH)

-(CH2)3- -SO2OH

SE-Sephadex Sephadex G-25

Wide-porous

Solid-Phase Extraction

Inorganic oxide and chemically bonded sorbents

47

based on molecular recognition sites are possible for specifically designed sorbent surfaces made selective for a particular analyte structural motif. The strong adsorption sites on unmodified inorganic oxides are generally associated with hydrogen-bonding interactions. These sites are generally deactivated by water which results in weak retention of analytes. Surface bonding of polar functional groups like cyano-, amine- or diol- provide an alternative to the harsh interactions with inorganic oxides for the extraction of polar compounds from organic solvents by hydrophilic interactions. Chemically bonded sorbents for reversed phase extraction are typically modified by octadecyl, octyl, cyclohexyl, or phenyl groups [10,12].

2.3.1

Sorbent pretreatment

Generally, SPE consists of four steps: column preparation, sample loading, column postwash, and sample desorption. The above mentioned prewash step is used for conditioning the stationary phase. Additionally, the postwash is utilized to remove undesirable substances. The target analytes are retained by the sorbent after washing out the interfering compounds. Particle-loaded membranes containing C18 is the main disk format in use. One of the drawbacks of disks compared with cartridges is the decrease in the breakthrough volume, mainly for more polar compounds. For this reason, disks are used when there is a strong interaction between the analyte and the sorbent [42]. Poole et al. [43] reviewed models and experimental procedures for predicting sorption isotherms (so-called breakthrough curves) for cartridge and disk devices. The breakthrough curves provide sufficient information for identifying the physicochemical processes involved in the solute transport through the packing materials.

2.3.2

Miniaturization and automation

Solid-phase extraction has benefited from and contributed to the general progress made in miniaturization and automation of extraction technique. Sampling in SPE occurs in two modes, off-line in which sample processing occurs before the chromatographic measurements and online in which sample processing is integrated directly with the chromatographic system. The latter is a more complex arrangement but has the advantages of lower contamination, automation, and higher sensitivity due to the analysis of the whole sample rather than an aliquot of the sample. Nowadays, online SPE coupled with either gas or liquid chromatography can be considered routine since only minor equipment requirements are necessary to fabricate a suitable system and commercial devices are available. The main advantages are higher throughput, good precision, limited manual processing, as well as lower cost, and greater sensitivity. As an alternative to conventional particle-packed extraction columns, Kang et al. described a packed-fiber SPE device for the analysis of trace pollutants in the environmental water and for drugs and their metabolites in human plasma [44]. They showed that nanofiber sorbents possess numerous advantages in terms of sensitivity, reproducibility, and

48

Solid-Phase Extraction

limited detection, and sample processing time. Recent years have witnessed the development of the microfluidic and on-chip analytical system with integrated solid-phase extraction units [45]. The microfluidic technology integrates injection, reaction, separation, and detection in a single device. Among different sample pretreatment methods carried out on a microchip device, SPE is one of the most important. A typical example is described by Kutter et al. [46] who prepared in an open-tube format a channel coated with C18 on a simple glass chip.

2.4

Sorbent characterization techniques

The properties and architecture of sorbents have a significant effect on their chemical modification. Those processes are investigated by the use of various physical and chemical techniques, such as elemental analysis, porosimetry, infrared spectroscopy, nuclear magnetic resonance imaging, microscopy, and tomography. Elemental analysis is one of the basic methods used to estimate the extent of surface coverage for chemically bonded sorbents from the percentage of carbon, hydrogen, and nitrogen, the surface area of the sorbent, and the structure of the bonded phase, usually expressed as mmols/m2 or the number of bonded ligands per nm2 [47,48]. However, the elemental analysis does not provide information about the homogeneity of the bonded phase and should be used in conjunction with other methods to infer sorbent properties. Elemental analysis has a precision of approximately 0.2% (w/w) for typical elements. Low-temperature nitrogen adsorption-desorption facilitates the determination of the quantity of gas adsorbed on the sorbent surface based on the mass and pressure of the analyzed gas. The BET adsorption isotherm provides information on the specific surface area (m2/g), volume (mL/g), and the diameter and size of pores. Owing to the size of nitrogen molecules, nitrogen adsorption is typically used for micropore and mesopore sorbents. Mercury porosimetry is more appropriate for macroporous sorbents. One of the most frequently used techniques for the determination of the adsorbent surface structure is infrared spectroscopy (IR). This method makes it possible to obtain information for bonded or associated silanol groups on the carrier surface and the surface structure of the chemically bonded phase. The main difficulty in applying this technique is the strong absorption of infrared radiation by silica gel. Advanced spectroscopic techniques, such as diffuse reflectance Fourier transform infrared spectroscopy (DRIFT) are preferable to transmission measurements [48e50]. The DRIFT method has been used to determine the hydrolytic stability of cyano, amino, and carboxyl groups added to the surface of silica gel. Thermogravimetry (TG), derivative thermogravimetry (DTG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) are used in the study of phase transformations. DSC and thermal analysis are valuable tools to determine the supramolecular organization of the solid phase. DSC experiments make it possible to precisely determine the interval in which the so-called transition phases occur.

Inorganic oxide and chemically bonded sorbents

49

They can be interpreted as the formation of more ordered structures, which is influenced by temperature or addition of an organic modifier, or ligands moving from unevenly thick clusters to a more homogenous monolayer. The reliability of the results is limited by the low density of the bonded phase coating. Thermogravimetric analysis, more often used for pure silica gel, provides valuable information for the temperature range in which the thermodesorption of physically adsorbed water or loss of mass connected with the condensation of silanols occurs. However, the characterization of SPE sorbents, involving the use of TGA allows conclusions to be drawn regarding the different location of ligands bound on the matrix surface. Microscopic methods support observations of the sorbent surface and any impurities adsorbed during synthesis. Pore diameter can be measured with sufficient magnification. The most popular microscopic methods include optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and scanning tunneling microscopy (STM) [51]. On the basis of fluorescence spectroscopy of surfaces modified with ligands containing fluorescent groups, Lochm€ uller [52] suggested three-component model for silica-based chemically bonded phases: (1) fluorophores which are not reached by solvent particles because they are localized in micropores or surrounded by an agglomeration of alkyl chains; (2) fluorophores open for interaction with solvent molecules; and (3) fluorophores relatively able to interact with solvent but bonded by hydrogen bonds to silanols present on the surface. A comparison of the results of XPS (a surface analysis technique with limited depth of several nanometers) and XRS enables conclusions to be drawn regarding the differences between the external layer of the bead and its internal pore network [53]. 29Si and 13C CP/MAS NMR is one of the most frequently used techniques providing detailed information on the presence of various ligands on the surface of a silica matrix and their conformation [47]. In contrast to infrared spectroscopy, NMR makes it possible to identify signals from different types of Si-OH groups. Initial problems connected with the limited mobility of ligands resulting in band broadening caused by chemical shift anisotropy and dipolar hydrogen-carbon interactions were solved by using magic angle spinning with dipolar decoupling. Mainly C18 sorbents and their modifications are used in biomedicine, pharmacology, or toxicology for extraction from different biological matrices. Martin et al. [54] studied the influence of different C18 silica-based sorbents on the extraction of acidic and basic compounds. The preferred sorbents were those of intermediate carbon loading. In addition, endcapping was important for compounds susceptible to silanophilic interactions. Buszewski et al. [55] prepared new types of silica gel sorbents used for five selected b-receptor antagonist sample preparation methods. They obtained successful preparation of silica gels of various porosities chemically modified by cholesterol ligands. Using optimized conditions, satisfactory recoveries of the analytes from buffers, urine, and blood samples were obtained. Several applications of these new modified silica sorbents are presented in Table 2.3 [56e65].

50

Table 2.3 Applications of new modified silica gel sorbents for SPE [56e65]. Target compounds

Adsorption capacity (mg/g)

Recovery

Ref.

Sorbent

Chelating agent

Sample

Silica gel

5-[(E)-(5-sulfonyl-1,3,4-thiadiazol-2yl) diazenil] pyrimidine-2,4,6 (1H, 3H, 5H)-trione

aqueous solutions

Th(IV)

24.85

e

[56]

Amino bonded silica gel

Polyelectrolyte

environmental water (drinking water, well water, snow water, waste water)

silver(I)

8.6

94%e105%

[57]

Silica gel

Phenothiazine

environmental water

nitrobenzene compounds

1.15 e 1.24

71.4%e124.3%

[58]

Silica gel

Pyrenebutyric acid

water

polychlorinated biphenyls

e

70.42e110.51

[59]

Silica gel

Glycerol

water

cobalt(II)

0.25

96.6e97.4

[60]

Aminopropylmodified silica gels

Aminopolycarboxylic

water samples

cobalt, nickel, cadmium, lead, zinc

e

99.4%

[61]

Solid-Phase Extraction

Octadecydimethylchlorosilane, dodecydimethylchlorosilane, Octydimethylchlorosilane, hexyldimethylchlorosilane, trimethylchlorosilane, octadecytrimethylchlorosilane

urine

SG-100

Monofunctional (MC18) and trifunctional (TC18) octadecylchlorosilanes

Silica gel Silica gel

[62]

5-hydroxyindole3-acetic acid (5-HIAA) and serotonin (5HT)

e

urine

glucuronide, paracetamol, cycteine and mercapturic acid

e

98%e110%, 1%e42%, 15%e120% and 5%e110%

[63]

Murexide

water

uranium (VI)

e

98%

[64]

2-((3-silylpropylimino) methyl) phenol

aqueous solutions

Fe(III), Pb(II), Cu(II), Ni(II), Co(II) and Zn(II) ion

69.8e90.0

95.6%e98.0%

[65]

15.2%e92.8% and 1.7%e75.3%

Inorganic oxide and chemically bonded sorbents

Kiesegel Si-60 and SG-100

51

52

2.5

Solid-Phase Extraction

Conclusions

The above overview demonstrates that sorbent selection makes it possible to isolate target compounds from various matrices by specific and nonspecific interactions. The effectiveness of the isolation process, i.e., selective sorption, and quantitative desorption, and as a consequence recovery, depends on the silica porosity and surface density of the bonded phase, and appropriate solvent selection. Interactions that participate in the process of isolating analytes from the matrix are characterized by varying energy. The selectivity of the process depends not only on the eluting power of solvents or their mixture but also on the selection of sorbent sorption properties. Trends in further development are currently focused on miniaturization and automation with a view to reducing costs, simplifying sample manipulation steps, and providing higher sample throughput. The application of micro- and nanoscale-based extraction and separation techniques will be developed in the future resulting in quick and sensitive analytical methods.

Acknowledgements The work was financially supported by the National Science Center, Cracow, Poland in the frame of the project Sonata 12 No. 2016/23/D/ST4/00305 (2017e2020) and Preludium 14 No. 2017/27/N/ST4/00354 (2018e19).

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[18] Ncube S, Madikizela L, Cukrowska E, Chimuka L. Trends Anal Chem 2018;99:101e16. [19] Muir D, Sverko E. Anal Bioanal Chem 2006;386:769e89. [20] Zhang L, Dou XW, Zhang Ch, Logrieco AF, Yang MH. Toxins 2018;10. https://doi.org/ 10.3390/toxins10020065. [21] Sheng H, Song Y, Bian Y, Wu W, Xiang L, Liu G, Jiang X, Wang F. Anal Methods 2017; 9:688e96. [22] Scott RPW. Silica gel and bonded phases. Their production, properties and use in LC. Chichester: J. Wiley & Sons; 1993. [23] Skoczylas M, Krzemi
nska K, Bocian Sz, Buszewski B. Silica gel and its derivatization for liquid chromatography. Ramtech Ltd. In: Meyers RA, editor. Encyclopedia of analytical chemistry. Applications, theory and instrumentation. Tarzana, CA: USA John Wiley & Sons, Ltd; 2017. p. 11450e71. Chapter 12. [24] Buszewski B, Jur
asek A, Garaj J, Nondek L, Nov
ak I, Berek D. J Liq Chromatogr 1987;10: 2325e36. [25] Sandoval J, Pesek J. J Anal Chem 1989;61:2067e75. [26] Tanaka N, Hashidzuma K, Araki M, Tsuchiya H, Okumo A, Iwaguchi K, Ochnishi S, Takai N. J Chromatogr 1998;448:95. [27] Gregg SJ, Sing KSW. Adsorption surface area and porosity. London: Academic Press; 1967. [28] Brunauer S, Emmet PH. J Am Chem Soc 1935;57:1754. [29] Brunauer S, Emmet PH, Teller E. J Am Chem Soc 1935;60:1938. [30] Berek D, Novak I. Chromatogr 1990;30:582. [31] Buszewski B, Leboda R. Chem Stos 1990;34:196. [32] Buszewski B, Jaroniec M, Staszczuk P, Gilpin RK. Wiad Chem 1995;49:223. [33] Nawrocki J, Buszewski B. J Chromatogr 1998;449:1. [34] Nov
ak I, Buszewski B, Garaj J, Berek D. Chem Papers 1990;44:31. [35] Schwarz H, Shaik S, Li J. J Am Chem Soc 2017;139:17201e12. [36] Unger KK. Packings and stationary phases in chromatographic techniques. 1990. Basel, New York.  [37] Zuvela P, Skoczylas M, Liu JJ, Bączek T, Kaliszan R, Wong MW, Buszewski B. Chem Rev 2018. https://doi.org/10.1021/acs.chemrev.8b00246. [38] Buszewski B, Jezierska M, Welniak M, Berek D. J High Resolut Chromatogr 1998;21: 267e81. [39] Galceran MT, Jauregui O. Anal Chem Acta 1995;304:75e84. [40] Petrova P, Karadjova I, Chochkova M, Dakova I, Karadjov M. Bulg Chem Comm 2017;SI E:95e100. [41] Kang X-J, Chen L-Q, Wang Y, Zhang Y-Y, Gu Z-Z. Biomed Microdevices 2009;11: 723e9. [42] Poole CF, Gunatilleka AD, Sethuraman R. J Chromatogr A 2000;885:17e39. [43] Luo D, Yu QW, Yin HR, Feng YQ. Anal Chim Acta 2007;588:261e7. [44] Oleschuk RD, Shultz-Lockyear LL, Ning Y, Harrison DJ. J Anal Chem 2000;72:585e90. [45] Kutter JP, Jacobson SC, Ramey JM. J Microcolumn Sep 2000;12:93e7. [46] Berendsen GE, Pikaart A, de Galan L. J Liq Chromatogr 1980;3:1437e64. [47] Albert K, Bayer E. J Chromatogr 1991;544:345e70. [48] Buszewski B, Kasturi P, Gilpin RK, Gandoga ME, Jaroniec M. Chromatogr 1994;39: 155e61. [49] Hetem MJJ. Chemically modyfied silica surfaces in chromatography e a fundamental study. Heidenberg: H€uthing Buch Verlag; 1993. [50] Sander LC, Callis JB, Field LR. Anal Chem 1983;55:1068e75.

54

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Solid-Phase Extraction

Murthy RSS, Crane LJ, Bronnimann CE. J Chromatogr 1991;542:205e20. Lochm}uller CH, Colburn AS, Hunnicutt ML, Harris JM. Anal Chem 1983;55:1344. Hunnicutt ML, Harris JM. Anal Chem 1986;58:748. Martin P, Morgan ED, Wilson ID. Anal Chem 1997;69:2972e5. Buszewski B, Welerowicz T, Tegowska E, Krzemi
nski TF. Anal Bioanal Chem 2009;393: 263e72. Kirkan B, Aycik GA. J Radioanal Nucl Chem 2016;308:81e91. Xiang G, Li L, Jiang X, He L, Fan L. J Chil Chem Soc 2013;4:2182e5. Peng XT, Zhao X, Feng YQ. J Chromatogr A 2011;1218:9314e20. Jin S, Zhang X, Gao R, He B, Yi Q. In: International conference on electric technology and civil engineering (ICETCE), Lushan; 2011. p. 5847e50. Safavi A, Iranpoor N, Saghir N, Momeni S. Anal Chim Acta 2006;569:139e44. Repo E, Warchoł JK, Sillanp€a M. J Sust Develop Energy Wat Environ Sys 2017;5: 89e100. Buszewski B. J Pharm Biomed Anal 1990;8:645e9. Mouelhi ME, Buszewski B. J Pharm Biomed Anal 1990;8:651e3. Sadeghi S, Sheikhzadeh E. J Hazard Mater 2009;163:861e8. Mortazavi K, Ghaedi M, Roosta M, Montazerozohori M. Ind J Sci Tech 2012;5: 1893e900.

Porous polymer sorbents

3


ria Fontanals, Rosa M. Marcé, Francesc Borrull Nu Department of Analytical Chemistry and Organic Chemistry, Universitat Rovira i Virgili, Tarragona, Spain

3.1

Introduction

Solid-phase extraction (SPE) is the most widely used technique for liquid samples or liquid extracts from other types of extractions. This is because SPE is versatile and can be used with different materials that cover various types of interactions with compounds having a wide range of properties. The first sorbents developed were silica-based sorbents and those modified mainly with long alkyl chains, such as C18 and C8, but also with phenyl, NH2 or others. However, these sorbents have some drawbacks, such as instability at extreme pHs, the activity of residual silanols, and low retention of polar compounds. Later, carbon-based sorbents appeared, including graphitized carbon blacks (GCBs) and porous graphitic carbon (PGC). However, they have a high retention capacity for some compounds, and thus eluting them is difficult and even irreversible. Polymer-based materials are one of the main developments with continuous progress appearing over the later years. This is mainly due to their morphological features (high surface area and well-defined porosity) and the diversity of synthetic routes, which facilitates incorporation of various chemical functionalities into the porous framework. This translates into a high retention capacity for different types of compounds and enhanced stability in different SPE conditions. These properties overcome the main disadvantages of silica- and carbon-based materials. Other recent developments in sorbent technology, including carbon nanomaterials, metallic nanoparticles, and metal-organic frameworks have also emerged [1], and will be covered in the other chapters of this handbook. However, over the years, porous polymer sorbents have remained an important class of sorbents for SPE. Porous polymer sorbents have been developed to accommodate high capacity extraction with the development of hypercrosslinked networks and/or the introduction of hydrophilic moieties, high selective extraction (the emergence of molecularly imprinted polymers (MIPs)) or a combination of these (mixed-mode ion-exchange materials). In this chapter, we cover all types of porous polymers, both commercially available and prepared in-house, except for MIPs, which are covered in a separate individual chapter. We give particular attention to the morphological and chemical properties of porous polymers, which are closely related to the type of polymerization procedure applied. The main technique used for polymer particle manufacturing is suspension polymerization, although precipitation polymerization (PP) and emulsion polymerization are also used. These polymerization techniques result in different

Solid-Phase Extraction. https://doi.org/10.1016/B978-0-12-816906-3.00003-0 Copyright © 2020 Elsevier Inc. All rights reserved.

56

Solid-Phase Extraction

morphological properties and different particle sizes. Therefore, the technique used should be selected carefully depending on the desired features of the polymer developed. Monolithic polymers are generated by bulk polymerization into the mold. Although porous polymers in particle format are more common in SPE, both particle and monolithic formats are covered in the different sections of this chapter. We also discuss the analytical capabilities of porous polymers and provide some illustrative examples of their applications.

3.2

Hydrophobic porous polymers

Porous polymeric materials were initially introduced to solve the drawbacks of silicaand carbon-based materials. From the early developments in the 1980s, research into hydrophobic porous polymers has aimed to make polymers more retentive toward target compounds.

3.2.1

Macroporous polymers

Conventional polymeric sorbents are crosslinked polymers obtained by suspension polymerization based on poly(styrene-divinylbenzene) (PS-DVB), which has a hydrophobic structure and specific surface area (SSA) of up to 500 m2/g. The amount of crosslinker (i.e., DVB) and the type of solvent (porogen) used during the suspension polymerization determine the porosity (IUPAC definition: micropores 50 nm) of the resin, which in turn determines the SSA (measured by N2 sorption and application of the BET theory). Thus, adding a thermodynamically good solvent for the monomer mixture leads to the formation of micropores and mesopores, resulting in beads with high SSAs. Adding a poor solvent leads to the formation of macropores, and thus beads with significantly lower SSAs [2,3]. When preparing macroporous polymers, adding thermodynamically good solvent (porogen) for the monomer mixture leads to the formation of micropores and mesopores, which results in beads with high SSAs. For example, using percentages of up to 70% DVB and toluene as the solvent (a good solvent for the PS-DVB polymer) produces macroporous PS-DVB sorbents with SSAs up to 600 m2/g; however, when cyclohexanone (a poor solvent) is used, a lower SSA (25 m2/g) is obtained [4]. In summary, macroporous SPE resins are formed when a porogen (toluene in this example) is present in the comonomer mixture (PS-DVB), which leads to phase separation of the polymer matrix at the stage where micropores are formed [2]. Moreover, generating beads by suspension polymerization usually results in uniformly shaped polymer particles with polydisperse sizes ranging from w50 to w500 mm, and these can be sieved later to obtain the suitable size for SPE. The hydrophobic structure of the PS-DVB polymer interacts with analytes through van der Waals forces and p-p interactions of the aromatic rings. Macroporous

Porous polymer sorbents

57

PS-DVB sorbents result in higher analyte retention compared to silica-based sorbents due to the numerous p-p sites they possess; however, the hydrophobic structure has a poor capacity and low retention for polar analytes [5]. Different macroporous hydrophobic sorbents are currently available: Amberlite XAD series supplied by Rohm & Haas (SSA from 100 to 800 m2/g), PLRP-S-10 (500 m2/g) and Bond Elut ENV (700 m2/g) supplied by Agilent; Strata SDB-L (500 m2/g) supplied by Phenomenex; AttractSPE DVB (600 m2/g) supplied by Affinisep; Purophase PCG900M (600 m2/g); and Isolute 101 (500 m2/g) supplied by Biotage. These sorbents have been used to extract various compounds, mainly those of moderate to low polarity. For instance, when online-SPE-liquid chromatography (LC) with tandem mass spectrometry (MS/MS) was used to determine 22 pesticides in groundwater, PLRP-S cartridges were used to retain 16 of the compounds, whereas a Hysphere GP resin (enhanced retention features) was used for the remaining six pesticides with recoveries higher than 75% in all cases with a 5 mL sample volume [6]. The first study of a polymer monolith for SPE was based on a macroporous ethylstyrene (ES)-DVB. This material was prepared directly inside a polyether ether ketone (PEEK) tube connected online to LC. In this case, the SSA of 400 m2/g was achieved by increasing the percentage of DVB (80%) while keeping the porogen in 52% dodecanol (poor solvent) and 8% toluene (good solvent) [7]. Svec [8] pointed out that there are considerable differences in pore sizes for identical polymerization mixture depending on whether bulk polymerization (monolith) or suspension polymerization (particles) is used. In addition, the type and proportion of porogens in the monolith preparation should be chosen carefully to ensure that the final monolith has suitable mesopores to enable the flow of different solvents employed in the extraction. Furthermore, it is always of interest to create micropores providing higher SSAs. More research into the effect of the porogen in monolith technology is required to determine the monolith morphology in more detail [9]. Nevertheless, toluene is often mixed with higher alcohols (i.e., dodecanol) to prepare PS-DVB monoliths, although this does not exclude the use of other porogens [9].

3.2.2

Hypercrosslinked polymers

The retention capacity of the polymer is closely related to the SSA. Although the percentage of the crosslinking agent is maximized in macroporous resins, a significant number of vinyl groups remain unreacted due to steric impediments of the aromatic groups. In the 1970s, Davankov presented a novel polymerization procedure to obtain polymers with hypercrosslinked (HXL) networks that consist of a postcrosslinking linear or lightly crosslinked (0.3%e2% DVB) PS by means of a FriedeleCrafts reaction using a crosslinking agent and a FriedeleCrafts catalyst (mainly FeCl3) [10]. With this procedure, almost all the aromatic rings can be crosslinked, thus yielding resins with an HXL structure, high micropore content, and ultrahigh SSA (up to 2000 m2/g). Unlike other porous polymers, HXL polymers have permanent porosity, which leads to extremely high SSA. These morphological properties give HXL resins a higher retention capacity than macroporous resins.

58

Solid-Phase Extraction

Later, Jer
abek [11] proposed including vinylbenzyl chloride (VBC) into the polymer chains, so that the chloromethyl (CH2Cl) moiety in VBC acts as an internal electrophile to crosslink the aromatic rings. Sherrington [12] presented a development that consisted of hypercrosslinking both gel-type and macroporous VBC-DVB precursors. In this approach, the crosslinking process is intramolecular, and thus, reactions are extremely efficient because a VBC-DVB precursor can become almost completely HXL in only 15 min [12]. In addition, if the precursor resin is already porous, the resulting HXL resins have a bimodal pore size distribution because they have both the original pore size (macroporous) and that generated during the hypercrosslinking process (microporous). Furthermore, Sherrington’s research group demonstrated that this approach is feasible when the precursors are obtained via suspension polymerization [13], which produces polydisperse particles with diameters ranging from 50 to 200 mm, nonaqueous dispersion (NAD) polymerization, which produces particles in the range 4e10 mm, and precipitation polymerization (PP), which produces monodisperse particles of w4 mm [14]. Later, Svec’s group also proposed this synthetic route in bulk polymerization to develop HXL in the form of monoliths [15]. Except in the case of monoliths (which should contain mesopores in order to ensure suitable kinetic and thermodynamic features), the different polymerization procedures produce similar morphological properties, and the main difference between the procedures is the particle size. Different reviews [16,17] detail the variation in the synthetic approaches as well as the morphological properties of the HXL HXL polymers have permanent porosity that leads to an extremely high SSA. The morphology of HXL monoliths is best illustrated in Ref. [18], describing the use of an HXL monolith to SPE. In this study, an HLX monolith was prepared from a precursor based on poly(VBC-S-DVB) and its morphology compared to a polyDVB monolith and PS-DVB in particulate form. It was shown that even the HXL monolith displayed larger SSAs (817 m2/g) than the polyDVB monolith (531 m2/g) in the dry state. However, in the solvated state, they found that the area of the HXL monolith decreased considerably (341 m2/g) whereas the area of the polyDVB monolith did not. Moreover, a comparison of the SPE performance of the polyDVB monolith and particulate poly(S-DVB) showed that anisole (as a model compound) was wellretained on the polyDVB monolith regardless of the flow rate (0.1e1 mL/min), but the particulate material behaved better at lower flow-rates. This was also confirmed when the polyDVB monolith was compared to different commercially available sorbents, including Bond Elut-LMS, Strata SDB, and Oasis HLB, which confirmed that the monolith’s performance is independent of the flow rate. Several manufacturers have commercialized HXL resins since they were first introduced as SPE sorbents. Some examples are: Amberchrom GC-161m (900 m2/g, SigmaeAldrich), Hysphere series (>1000 m2/g, Spark Holland), Hypersol-Macronet sorbents (1200 m2/g, Purolite International), Lichrolut EN (1200 m2/g, Merck), EnviChrom P (900 m2/g, Supelco), and Chromabond HR-X (1200 m2/g, MachereyeNagel).

Porous polymer sorbents

59

Other advances in HXL technology for SPE have been made over the years [13,14,19]. In one of these studies [19], three different HXL polymers were synthesized from precursors obtained by precipitation polymerization with areas ranging from 880 to 1320 m2/g depending on the percentage of VBC feed in the precursor (from 25% to 75%). The performance of these three HXL sorbents was compared to a commercially available HXL sorbent, Lichrolut EN (1200 m2/g), for the online extraction of phenols and pesticides from environmental water samples. The results showed that the in-house prepared HXL sorbents had a higher extraction efficiency and capacity (they attained recoveries between 74% and 105% for a 500 mL sample volume) compared to Lichrolut EN (recoveries of 41%e78%). As all the sorbents had similar morphological (SSAs w 1000 m2/g) and chemical properties (hydrophobic), the in-house prepared sorbents’ higher extraction efficiency was attributed to the particle size. The particle size was w4 mm for the in-house prepared sorbents, compared to 40e120 mm for Lichrolut EN. Smaller particle sizes promote better packing in the sorbent bed (especially in online SPE) and lead to a more efficient and reproducible extraction procedure. Five different sorbents (Chromabond HR-X, Oasis HLB, Bond Elut Plexa, SampliQ C18, and Chromabond HR-HAW) were evaluated for the extraction of 24 compounds (including pharmaceuticals, personal care products, endocrine disrupting compounds and artificial sweeteners) by offline SPE/LC-MS/MS in water samples. The hydrophobic Chromabond HR-X was the most effective for the compounds studied (recoveries >60%) for all samples analyzed (groundwater, surface water, and raw water). This sorbent was particularly successful for the sweeteners, which are the most challenging compounds in the group [20]. Further examples of commercial HXL sorbents used to extract different compounds, such as UV filters [21] or sweeteners [22] from environmental water samples have been reported. Nevertheless, there are some applications for other fields. For instance, Lichrolut EN (80 mg) was used for the online extraction of a group of endocrine disrupting compounds from 5 mL of urine, 1 mL of blood, and 1 mL of breast milk with good results achieved [23]. It should be noted that although the above analytes have polar functional groups, they can only be retained on the sorbent through hydrophobic interactions. In view of this, introducing polar features into these sorbents might improve the retention of polar compounds.

When hydrophobic HXL sorbents are used in SPE, the analytes can only be retained on the sorbent through hydrophobic interactions.

3.3

Hydrophylic porous polymers

Introducing polar moieties into a sorbent promotes polar interactions with the analytes, and thus enhances the retention capacity of the sorbent as well as reducing its high hydrophobicity. Hydrophilic porous polymers can be prepared by chemical modification of the hydrophobic network or by copolymerization with hydrophilic monomers.

60

Solid-Phase Extraction

The following sections cover these two strategies for commercially available and in-house sorbents. Hydrophilic porous polymers can be prepared by chemical modification of the macroporous or hypercrosslinked hydrophobic network with polar moieties or by copolymerization with hydrophilic monomers.

3.3.1

Functionalized polymers with polar moieties

Generally, the PS-DVB polymeric network is chemically modified by means of a FriedeleCrafts reaction. With this approach, polymeric materials with optimal morphological features can be obtained from the precursor. Fig. 3.1 shows different polar functional groups, such as acetyl [24,25], hydroxymethyl [24], benzoyl [26], o-carboxybenzoyl [27], 2-carboxy-3/4-nitrobenzoil, and 2,4-dicarboxybenzoyl [28], that have been introduced into an HXL PS-DVB network. The main limitation of this approach is the low degree of modification due to the restricted accessibility of the functional group in the crosslinked matrix. In this sense, the degree of modification for the benzoyl moiety was 60%, whereas it was only 6% for 2,4-dicarboxybenzoyl (larger moiety) due to restricted accessibility of the reactive sites [28]. However, the polymers chemically modified with polar moieties showed higher retention of polar compounds during extraction than their unmodified analogs. More recently, Amberlite XAD-4 was functionalized with chelating agents, so it could be used for the preconcentration of metal ions from aqueous solutions [29]. It is noteworthy that, in all instances, the precursor polymer has the same morphological and particle size characteristics after the functionalization.

Hydroxymethyl

Acetyl

Benzoyl

Chemical modification PS-DVB o-Carboxybenzoyl

o-Carboxy-3/4-nitrobenzoyl

Pyrrolidone

Figure 3.1 Scheme of some chemical modifications of PS-DVB polymer.

Porous polymer sorbents

61

Several commercial chemically modified sorbents, both macroporous and HXL, are available: Strata-X (800 m2/g, Phenomenex), Bond Elut Plexa and Bond Elut Focus (550 m2/g, Agilent Technologies), HyperSep Retain PEP (550e750 m2/g, Thermo Scientific), ExtraBond PolyU and EB2 (850 and 700 m2/g, respectively, Scharlau). A more exhaustive list of these sorbents and their features is provided in previous reviews [5,30]. Bond Elut PPL, Bond Elut ENV, and Strata X were compared for the extraction of trihalomethane precursors from water samples. All three sorbents were found to be suitable for onsite monitoring of trihalomethane precursors since they all successfully extracted trihalomethanes [31]. In another example, Strata X was used for the clean-up step during the extraction of antibiotics from porcine edible tissues [32]. Another study evaluated different strategies and different SPE sorbents, such as Oasis HLB and Bond Elute Nexus; Bond Elut Plexa as the clean-up sorbent for analysis of cephalosporins and quinolones in milk [33]. Some approaches have been conducted in monoliths to functionalize the hydrophobic network with polar moieties, although most of these materials act as ion-exchangers (summarized in Section 3.4). In one study [34], poly(DVB) was grafted with two polyethylenglycol dimethacrylate (pEDMA) monomers with different molar mass distributions (Mn360 and Mn950) to isolate analytes from protein-rich samples. One of the critical points during the preparation was optimizing the percentage (from 5% to 20%) of grafting monomer so that its density was balanced to prevent the proteins from interacting with the hydrophobic surface (DVB) but sufficiently thin and permeable to ensure that the analyte would interact with the inner surface. Finally, poly90%DVB-g-10%pEDMA950 showed the best performance [34].

3.3.2

Copolymerization with a hydrophilic monomer

This approach consists of copolymerizing a hydrophilic monomer with a crosslinking monomer. The hydrophilic monomer favors hydrophilic interactions, whereas the crosslinking monomer, usually DVB, increases the SSA and favors hydrophobic interactions. This strategy has been widely applied to both commercial and in-house SPE sorbents. In most cases, these hydrophilic copolymers are obtained by suspension polymerization, which produces macroporous materials with 500e800 m2/g SSAs and particle sizes ranging from w50 to 200 mm. In hydrophilic sorbents prepared by copolymerizing a hydrophilic monomer with a crosslinker, the hydrophilic monomer favors the hydrophilic interactions, whereas the crosslinking monomer increases the SSA and favors the hydrophobic interactions. Different hydrophilic sorbents with macroporous structures have been commercialized: Amberlite XAD-7 and XAD-8 (methacrylic acid (MAA)-DVB, 450 and 310 m2/g, respectively) supplied by Rohm and Haas, and Bond Elut Nexus (no data) supplied by Agilent Technologies; Sep-pak Porapak RDX (N-vinylpyrrolidone

62

Solid-Phase Extraction

(NVP)-DVB, 550 m2/g) and Oasis HLB (NVP-DVB, 830 m2/g) supplied by Waters, or Chromabond HLB (NVP-DVB, 750 m2/g) supplied by MachereyeNagel, or Extrabond EHB (no data) from Sharlau. Discovery DPA 6S (Polyamide (PA)-DVB, no data) and SampliQ OPT (PA-DVB) are both from Agilent Technologies. Table 3.1 shows selected examples of the commercially available hydrophilic sorbents. More information is provided in previous reviews [5,30]. Oasis HLB is the most widely used commercially available sorbent, used to extract different types of compounds (i.e., pharmaceuticals, proteins, pesticides, sweeteners, high volume production chemicals, illicit drugs, etc.) from matrices, such as food, environmental waters, and biological fluids and tissues [5,30]. Moreover, it has been used not only to enrich analytes from the matrix but also eliminate matrix interferences from complex samples. In order to accommodate all types of applications, the commercially available sorbents are presented in different formats (cartridge, precolumns, 96-well plate, disks) with different bed-volumes (usually, from 30 to 500 mg). For instance, Oasis HLB in a cartridge format (150 mg) was used in the multiresidue screening of around 1500 organic pollutants from 250 mL of environmental water [40] with a preconcentration factor of 500. In contrast, a 96-well plate format (10 mg) was used for the cleanup of different growth promoters from 0.5 g of bovine meat, which was washed six times to remove the matrix interferents [41]. In addition, different comparative studies were conducted to select the best SPE sorbent. For example, a study compared the performance of 11 SPE sorbents, including polymeric (Oasis HLB, SampliQ OPT, SampliQ Polymer SCX, Strata SDB-L, and Strata-X) and silica-based (Sep-Pak C18, SampliQ C8, SampliQ C8/Si-SCX, SampliQ C18, Strata C8, and Strata C18) sorbents, for the enrichment of a group of pharmaceuticals from 100 mL of surface water [42]. The recovery (%R) values for the different SPE sorbents showed that polymeric sorbents perform better and have a higher retention capacity than silica-based sorbents. Of the polymeric sorbents, Strata-X (recovery values > 95%), Strata SDB-L (%R > 86%) and Oasis HLB (%R > 99%) provided the best SPE performances. This can be attributed to the presence of DVB, which promotes p-p interactions and polar moieties, thus enhancing the retention of the studied analytes. Therefore, when the porous material contained polar moieties, both functionalization and copolymerization had similar outcomes. Although several hydrophilic sorbents are commercially available, several research groups have synthesized alternative copolymeric hydrophilic sorbents with different hydrophilic monomers and degrees of crosslinking. Our research group has synthesized most of the in-house hydrophilic copolymeric sorbents, but the Bagheri and Trochimczuk research groups have also been working in this area. Table 3.1 provides some examples of the in-house synthesized hydrophilic copolymeric sorbents. A series of conductive linear polymers (without crosslinking agent) based on polyaniline (PANI) [35], poly-N-methylaniline (PNMA) [43], polydiphenylaniline (PDPA) [43], and polypyrrole (PPy) [44] have been synthesized. As they are linear polymers, their SSAs are limited (32e48 m2/g), which is possibly the cause of the low recoveries achieved for the extraction of polar compounds. In another study [45], copolymers were prepared from di(methacryloyloxymethyl) naphthalene (DMN), p,p’-dihydroxydiphenylmethane diglycidil methacrylic ester (MEMDE),

Table 3.1 Structure and properties of some sorbents obtained by copolymerization with hydrophilic monomer. Sorbent

Methacrylate-divinylbenzene (MA-DVB) xl *

*

Amberlite XAD-8

O

O

Bond Elut Nexus Oasis HLB

N-vinylpyrrolidone-divinylbenzene (NVP-DVB)

Porapak RDX

*

Chromabond HLB SampliQ OPT

Polyamide-DVB *

450

Applied separations

575

Agilent Technologies

830

Waters

550

N

O

Supplier/reference

310

CH

*

SSA (m2/g)

xl

750

MachereyeNagel

n.d.

Agilent Technologies

48

[35,36]

460

[37]

560

[37]

Porous polymer sorbents

Amberlite XAD-7

Copolymeric structure

*

NH O

PANI

R¼H; polyaniline (PANI)

R N

*

AN-DVB MAN-DVB

R¼H3; acrylonitrile (AN)-DVB R¼CH3; methacrylonitrile (MAN)-DVB

R xl *

*

n *

C N

NVIm-DVB

N-vinylimidazole-DVB

626

[38]

MAA-EDMA

Methacrylic acid-ethylene dimethacrylate

50

[39] 63

n, linear polymer; n.d., no data; xl, crosslinked.

64

Solid-Phase Extraction

p,p0 -dihydroxydiphenylpropane diglycidil methacrylic ester (MEDDE) and bis(maleimido)diphenylmethane (BM), all polymerized with DVB; however, although they are polar, they have low SSAs (up to 100 m2/g) due to the low DVB content. More recently, 2-hydroxyethyl methacrylate (HEMA) copolymerized with the hydrophilic crosslinking agent EDMA, was synthesized and applied as a sorbent in a tip connected to a syringe to extract antibiotics from milk. However, its morphological and polar features were not described, and therefore, it cannot be compared [46]. The Trochimczuk group also prepared copolymers based on acrylonitrile (AN) [37], methacrylonitrile (MAN) [37] and cyanomethylstyrene (CMSt) [47], which are all hydrophilic monomers crosslinked with DVB. These sorbents had larger SSAs (300e700 m2/g) due to a higher amount of DVB. Moreover, the authors concluded that both the hydrophilic content and the SSA are equally important for retaining polar compounds. Therefore, copolymers with a 1:1 ratio provided the best performance. A proper balance between hydrophilicity (from the polar monomer) and SSAs (from the crosslinker) was also demonstrated in studies aimed at the development of new hydrophilic copolymeric sorbents for SPE. These sorbents were based on 4-vinylpyridine-divinylbenzene (4VP-DVB) [48,49], N-vinylimidazole-divinylbenzene (NVIm-DVB) [38,50] and 4-vinylimidazole-divinylbenzene (4VIm-DVB) [51]. They were used to extract different polar compounds from environmental water samples. Table 3.2 summarizes the recovery for different sample volumes of ultrapure water spiked with phenolic compounds for different in-house prepared macroporous hydrophilic copolymer sorbents. Apart from the ratio of the crosslinking agent, the type and proportion of the porogen also plays an important role in the pore formation, and eventually, in the SSA. Nonetheless, there are no exact rules for selecting the polymerization conditions for the monomer system, and each system should be evaluated individually. In most cases, toluene was used as the main component of the preferred porogens because DVB or other related styrenic monomers are used in the copolymerization [54]. For instance, in the preparation of 4VP-DVB resins [48], as 4VP is a styrenic monomer, the porogen system was the same as that used in the preparation of PS-DVB consisting of toluene (88%) and dibutyl phatalate (12%). However, when dibutyl phatalate was replaced by cyclohexanol (a poor solvent for PS-DVB) the SSA dropped from 115 to 88%) than for Lichrolut EN (>79%) Oasis HLB (>66%) or Isolute ENVþ (>80%). A triphenylamine-based HXL polymer was prepared using self-condensation of triphenylamine in a typical FriedeleCrafts reaction. The polymer attained an SSA of 720 m2/g with the presence of nitrogen moieties; thus, this sorbent presented both hydrophobic and hydrophilic interactions and was evaluated for the extraction of pesticides from food samples [55]. It should be noted that most of the monoliths applied in sample preparation are based on the copolymerization of at least one, but often two, hydrophilic monomers. Table 3.3 lists some examples of the main features for monolith preparation based on hydrophilic monomers. EDMA (polar monomer) is the most commonly used crosslinker [56e59,64e67], although DVB is also used [34,68,69]. These crosslinkers are copolymerized with different polar monomers, such as glycidyl methacrylate (GMA) [59,67e69], MAA [57,65], or butylmethacrylate (BMA) [56,66], or polymers in the form of termonomers, by combining two functional monomers, such as GMAMAA-triethylene glycol dimethacrylate (TEGDMA) [70] or GMA-S-DVB [68,69]. Moreover, the ratio of the different monomers varies (details in Table 3.3). This ratio affects not only the composition of the materials but also the pore formation. The mixture of cyclohexanol and dodecanol is popular as a porogen for thepreparation of hydrophilic monoliths, especially those based on GMA-EDMA [9,62,67,71]. However, other binary mixtures, such as 1-propanolol and 1,4-butanediol [56,60,62,64,72] or single porogens 1-propanolol [61] or n-hexanolol [73] have been used. Monoliths have also been prepared using polyethylene glycol (PEG) with a different molecular weight in combination with alcohols [58,59,65,66]. In addition, the higher the PEG molecular weight, the larger the pore size [9]. As stated before, the choice and the proportion of these porogens affect the morphology of the monolith. For example, different porogens were selected to prepare alkyl methacrylate-based monoliths by copolymerization of EDMA (crosslinker) and GMA, BMA, or LMA to improve the final porosity. Cyclohexanol and 1-dodecanol were selected to prepare a GMA-EDMA monolith; however, this monolith did not have large SSAs (due to the lack of mesopores and micropores), and thus, had a low loading capacity. The BMA-EDMA and LMA-EDMA monoliths were prepared using 1-propanolol and 1,4-butanediol as porogens, providing higher porosity [62]. Monolithic polymers were prepared in different molds and used accordingly in different SPE formats, including columns [58e60,63,65e67,71,73] that were later connected online to LC, syringes or cartridges [18,34,57,61,62,64], or pipette tips [56,68], among others. Table 3.3 summarizes the information on the monolith format.

SSA (m2/g)

Format

References

340e817

Bulk PE tubing PP cartridges

[18]

1-Propanolol 65% 1,4-Butanediaol 25% Water 10%

90

0.014e0.041 mg/L

Adsorbent was conditioned with (2 x 3) mL of MeOH and deionized water. Extraction capacity of mMWCNTs, three nonmagnetic polymeric sorbents and three magnetic polymeric sorbents was compared.

[28]

3 steroid hormones

Milk (40 mL)

m-mdSPE

Fe3O4@ SrTiO3@ MWCNTs (4 mg)

HPLC-VWD

60e113

0.03e0.34 mg/L

Milk samples were previously deproteinized with 1:1 milk:ACN volume ratio. Sorbent was synthesized with 1:1 w/w ratio of m-MWCNTs:SrTiO3.

[29]

Continued

Table 4.2 Applications of carbon nanotubes in solid-phase extraction.dcont’d Sample (amount)

SPE mode

Sorbent (amount)

Determination technique

Recovery (%)

Detection limits

6 Sudan dyes and Para Red

Chili powder, chili tomato sauces and ketchup (10 g)

m-mdSPE

g-Fe2O3@ MWCNTsCOOH (40 mg)

HPLC-DAD

-

18 chiral pesticides

Water (200 mL) Soil and river sediments (2 g)

m-mdSPE

Fe3O4@ MWCNTsNH2 (75 mg)

UHPLC-QqQMS/MS

80e106

Analyte

Comments

Reference

0.13e0.84 mg/L

Samples were pretreated with 30 mL of acetone: DCM:MeOH (3:2:1, v/ v/v), diluted with ACN:MeOH (80:20, v/v) to 50 mL, and the extract was submitted to the m-m-dSPE procedure.

[30]

0.02e0.62 ng/L

Solid samples were pretreated with 10 mL of ACN and 10 mL of ultrapure water. 10 mL of supernatant was diluted with 50 mL of ultrapure water and the extract was submitted to the m-m-dSPE procedure. Pristine MWCNTs and MWCNTs-NH2 were compared.

[31]

5 mycotoxins

Salviae miltiorrhizae Radix et Rhizoma (Danshen) (2 g)

m-SPE

Fe3O4@ MWCNTs (20 mg)

UHPLC-QqQMS/MS

74e92

0.45e1.80 mg/kg

Samples were pretreated with 10 mL of ACN: water (84:16, v/v). 5 mL of supernatant was dried, reconstituted in 0.5 mL of ACN:water (20:80, v/v) and diluted with 4.5 mL of water. The extract (5 mL) was submitted to the m-SPE procedure. Fe3O4 enhanced the adsorption ability of MWCNTs.

[32]

7 PCBs

Environmental water (1 mL)

m-dSPE

MWCNTsCOOH (2 mg)

GC-ECD

99e106

1.4e3.5 ng/L

SWCNTs, MWCNTs, and MWCNTs-COOH were compared.

[33]

Continued

Table 4.2 Applications of carbon nanotubes in solid-phase extraction.dcont’d Analyte 9 PAHs, Cr(II), Cd(III) and Pb(II)

Sample (amount)

SPE mode

Sorbent (amount)

Determination technique

Recovery (%)

Detection limits

Water (250 mL)

m-dSPE

MWCNTsCOOH (50 mg)

(i) GC-Q-MS (ii) AAS

(i) 68e102 (ii) 75e88

(i) 0.003 mg/L (ii) 0.030 mg/ L

Comments

Reference

After extraction procedure, the mixture was transferred to a PTFE column containing two PTFE frits, and MWCNTs were dried under vacuum. (i) PAHs were reextracted with 8 mL of DCM and analyzed by GC-MS. (ii) Metal ions were reextracted with 5 mL of HNO3 and analyzed by AAS. Helical MWCNTs, MWCNTs (> 50 nm), MWCNTs (< 8 nm), MWCNTs-OH, and MWCNTs-COOH were tested as sorbents.

[34]

Cr(III)

Water (200 mL)

m-mdSPE

Fe3O4@ MWCNTsCOOH (50 mg)

Laser-induced fluorescence spectroscopy detection

91e102

0.094 nM

Cr(III) analysis were performed in a flow injection microfluidic chip. Rhodamine derivative as fluorescent derivatizing agent. Total chromium was determined as Cr(III) by reducing Cr(VI) to Cr(III).

[35]

18 OCPs

Environmental water (50 mL)

m-mdSPE

ChCl/urea (DES) @Fe3O4@ MWCNTs (-)

LVI-GC-mECD

91e102

0.04e0.27 ng/L

Bucky gel sorbent was formed by 5 mg of mMWCNTs dispersed in 50 mL of ChCl/urea DES. Analytes were reextracted from mMWCNTs with 150 mL of ACN. Equilibrium time for analytes extraction had no significant effect on the extraction efficiency.

[36]

m-dSPE: micro-dispersive solid-phase extraction; m-ECD: micro-electron capture detector; m-SPE: microesolid-phase extraction; 8-iso-PGF2a-d4: 8-iso-prostaglandin-F2a-d4; AAS: atomic absorption spectrometry; ACN: acetonitrile; CE: capillary electrophoresis; ChCl: choline chloride; CRM: certified reference material; DAD: diode-array detector; DCM: dichloromethane; DLLME: dispersive liquideliquid microextraction; ECD: electron capture detector; FAAS: flame atomic absorption spectrometry; GC: gas chromatography; HPLC: high-performance liquid chromatography; ICP: inductively coupled plasma; LC: liquid chromatography; LDH: layered double hydroxide; LVI: large-volume injection; m-m-dSPE: magnetic-microdispersive solid-phase extraction; m-dSPE: magnetic-dispersive solid-phase extraction; MeOH: methanol; m-MWCNT: magnetic-multiwalled carbon nanotube; MS/MS: tandem mass spectrometry; MS: mass spectrometry; MWCNT: multiwalled carbon nanotube; MWCNT-COOH: carboxylated-multiwalled carbon nanotube; MWCNT-NH2: amine functionalized-multiwalled carbon nanotube; MWCNT-OH: hydroxylated-multiwalled carbon nanotube; NSAID: nonsteroidal antiinflammatory drugs; OCP: organochlorine pesticide; o-MWCNT: oxidized-multiwalled carbon nanotube; PAH: polycyclic aromatic hydrocarbon; PCB: polychlorinated biphenyl; PS-DVB: poly-(styrene-co-divinylbenzene); PTFE: polytetrafluoroethylene; Q: single quadrupole; QqQ: triple quadrupole; QTrap: triple quadrupole linear ion trap; SFOD: solidified floating organic droplets; SPE: solid-phase extraction; SWCNT: single-walled carbon nanotube; ToF: time-offlight; UHPLC: ultrahigh-performance liquid chromatography; UV: ultraviolet detector; VWD: variable-wavelength detector; wt: weight.

98

Solid-Phase Extraction

oxidation [21] or functionalized with diphenylcarbazide [24], pyridine [25], bovine serum albumin [26], poly-L-methionine [27], poly-(benzyl methacrylate) [28]; and in combination with m-NPs [29] or graphene oxide (GO) [30] for the extraction of metal ions [24e27], pesticides [23], F2-isoprostanes [21], PAHs [28], mycotoxins [29]; and for drugs [30] from plant sources [24,29], environmental waters [23,24,27,28], fruits [25], blood serum [26], cord and maternal plasma [21], fish oil dietary supplements [27], and pork [30] using a conventional packed cartridge SPE format. For use in dSPE, CNTs have been used alone [31,32], after functionalizing with hydroxyl [33] or carboxyl groups [34,35], with organic molecules, including crown ethers [36], or in combination with other nanomaterials, such as layered double hydroxide (LDH) [37]. They have been used for the extraction of a wide variety of organic and inorganic compounds including antibiotics [32], fungicides [33], mycotoxins [31], nonsteroidal antiinflammatory drugs (NSAIDs) [37], metal ions [38], dyes [36], polychlorinated biphenyls (PCBs) [34], and PAHs [35] from water [31e35,37], fruit juices [33], infant milk [31], plasma [37], urine [37], rice [38], tomato sauce [36], and salted duck egg yolk [36], among others. As an example, Arghavani-Beydokhti et al. [37] developed a nanohybrid material formed by oxidized MWCNTs (o-MWCNTs) and a Zn-Al-LDH for m-dSPE for the extraction of three NSAIDs from plasma, urine and wastewater samples. In order to avoid the characteristic centrifugation steps of dSPE procedures, m-dSPE coupled to solidified floating organic droplets (SFOD)-dispersive liquid-liquid microextraction (DLLME) was carried out. The first step consisted of the ultrasound-assisted extraction of 10 mL of aqueous sample solution with 8 mg of Zn-Al-LDH@o-MWCNTs. To re-extract the NSAIDs, 100 mL of trifluoroacetic acid (TFAA) (8% v/v) was sucked into the syringe holding the sample, and the plunger slowly moved to fully dissolve the Zn-Al-LDH. Afterward, the residue was transferred to a glass tube, diluted to 5 mL with deionized water and submitted to the SFO-DLLME method using 60 mL of 1-undecanol as the extraction phase. The mixture obtained was transferred to another tube containing 5 g of NaCl to induce the salting-out effect. The 1-undecanol floating drops with extracted target analytes were solidified in an ice bath, collected with a spatula, melted at room temperature, and analyzed by LC. The sample preparation procedure was completed in 8 min without using centrifugation with a recovery range of 90%e103% and LODs of 0.1e0.2 mg/L. One of the most relevant modifications employed in recent years is the combination of CNTs with m-NPs, not only because it improves the dispersion of the carbonbased nanomaterials in solvents, but also because it greatly simplifies the extraction procedure compared to conventional SPE and dSPE. In addition to its use with m-NPs (both magnetite [39e42] and maghemite [43]) to carry out magnetic dSPE (m-dSPE), its combination with other materials, such as graphene [44] or reducedGO (r-GO) [45], SrTiO3 [40] or even deep eutectic solvents (DESs) [46], and also functionalization, including carboxyl [42,43] and amino [41] groups or poly(styreneco-divinylbenzene) (PS-DVB) [39] have been used. These sorbents were applied for the extraction of drugs [39,45], steroid hormones [40], dyes [43], pesticides [41,46], PAHs [44], and metal ions [42] from urine [39], milk [40,45], chili powder [43] and

Carbon-based adsorbents

99

tomato [43], ketchup [43], water [41,42,46], soil [41] and sediments [41], among others. Yousefi et al. [46] combined the favorable characteristic properties of MWCNTs and m-NPs, with a new type of environmentally friendly solvents, namely DESs. In this way, they created a bucky gel with increased dispersibility of magnetic MWCNTs (m-MWCNTs) by combining them with a DES prepared from choline chloride (ChCl) and urea. In addition, ChCl can act as a salting-out agent, enhancing the extraction efficiency of the procedure. The above sorbent was used for the extraction of 18 organochlorine pesticides (OCPs) from environmental water samples for determination by GC For the m-m-dSPE step, 5 mg of m-MWCNTS were dispersed in 50 mL of DES for the extraction of up to 50 mL of the sample solution. The m-MWCNTs were isolated by an external magnet, and the analytes desorbed with 150 mL of acetonitrile (ACN) for analysis. Recovery values between 91% and 102% and LODs of 0.04e0.27 ng/L were suitable for the determination of OCPs in environmental water samples. Other applications of CNTs in SPE involve their use as a clean-up sorbent in the QuEChERS (quick, easy, cheap, effective, rugged, and safe) method [47] to reduce matrix effects caused by co-extractive materials from food samples.

4.2.3

Graphene

Graphene, a nanomaterial consisting of a carbon monolayer packed in a bidimensional honeycomb network, is considered the basic building block of carbon-based materials including graphite, fullerenes, and CNTs, etc [48]. First isolated in 2004 [49], it has become of great interest in many scientific fields, mainly due to its ultrahigh theoretical surface area (2630 m2 g
1 for a single layer); high mechanical strength, elasticity, and thermal conductivity; and high electron conductivity [50]. In particular, graphene has played an important role in analytical methods due to its high hydrophobicity and ease of functionalization, as well as its electron delocalization that allows strong p-p interactions with benzene-ring structures [10,51]. In addition, its planar structure provides graphene a high adsorption capacity with analyte adsorption able to occur on both sides of the sheet [52]. These features make it possible to obtain high extraction efficiencies using small amounts of graphene as a sorbent [51]. Apart from that, graphene can be easily obtained by oxidation of graphite without the formation of residual heterogeneous materials derived from catalytic processes [8]. As shown in Table 4.3, graphene-based sorbents have been used in several SPE formats for the analysis of both organic compounds [45,53e63] and metal ions [64e66] in environmental [53,54,58,60,64,66], food [45,55,57,59,61e63,65] and biological matrices [56]. On account of its tendency to agglomerate (that reduces the theoretical surface area) and its poor dispersibility in water, raw graphene has not been widely used as a sorbent for SPE [67,68]. However, GO, which contains large quantities of oxygen atoms as epoxy, hydroxyl, and carboxyl groups on its surface has attracted more attention as a sorbent for extraction since it shows a better dispersibility in aqueous solutions and additional capability to establish hydrogen bonds and electrostatic interactions with organic compounds and metal ions [51]. Similarly, r-GO, which is an

Table 4.3 Applications of graphene-based materials in solid-phase extraction. SPE mode

Sorbent (amount)

Determination technique

Recovery (%)

Detection limits

Tap, river, lagoon water, and wastewater (200 mL)

SPE

GO@P-pAP (150 mg)

FAAS

97e98

Trout, salmon (200 mg) River and sea water (100 mL)

m-

GO@Phen (1 mg)

ICP-OES

7 PAHs

Tap, river, well, and wastewater (50 mL)

m-dSPE

Graphene@ MOF199 (20 mg)

3 NSAIDs

Tap, river water, and groundwater (10 mL)

m-mdSPE

Fe3O4@ PEI-rGO (5 mg)

Analyte

Sample (amount)

Ni(II)

Pb(II)

dSPE

Comments

Reference

0.7 mg/L

A comparison with GO was carried out. The sorbent could be reused for seven cycles. Accuracy and precision of the method was tested by analyzing a CRM.

[53]

96e104

0.046 mg/L

Fish samples were aciddigested with nitric acid and the extract was submitted to the m-dSPE extraction procedure. Accuracy and precision of the method was tested by analyzing CRMs.

[54]

GC-FID

92e100

0.003-0.01 mg/L

A comparison with MOF199 and Fullerene@ MOF-199 was carried out.

[55]

HPLC-DAD

91e101

0.2 mg/L

A comparison with Fe3O4@PEI, Fe3O4@rGO and Fe3O4@PEIGO was carried out. The sorbent could be reused for 10 cycles.

[56]

Imidacloprid and 2, 4-dichloro phenoxyacetic acid

Tap water (50 mL) Cucumber and tomato (1 g)

m-mdSPE

TPN@ Fe3O4@ GO (11 mg)

HPLC-UV

91e102

0.17e1.7 mg/L

Vegetable samples were previously extracted using ACN/water and the extract was submitted to the m-mdSPE extraction procedure. The m-m-dSPE procedure was optimized using an experimental design.

[57]

SAs

Milk (10 mL)

m-mdSPE

Fe3O4@rGOCNTs (20 mg)

HPLC-UV

88e106

0.35e1.32 mg/L

Milk samples were deproteinized before mm-dSPE the extraction procedure. The m-m-dSPE procedure was assisted by vortex.

[46]

4 PFAAs

Blood serum (0.5 mL)

SPE

GO-NH2@ S-DVB (500 mg)

IC-Q-MS

86e101

-

A comparison with commercial HLB sorbent was carried out. TFAA was used as surrogated. Blood serum samples were acid-digested and deproteinized before the SPE extraction procedure. The sorbent could be reused for five cycles. LOQs were found in the range 0.50e2.0 mg/L. The method was applied to real samples.

[58]

Continued

Table 4.3 Applications of graphene-based materials in solid-phase extraction.dcont’d Analyte

Sample (amount)

SPE mode

Sorbent (amount)

Determination technique

Recovery (%)

Detection limits

9 Mycotoxins

Milk (1 mL)

m-SPE

r-GO@Au (10 mg)

UHPLC-QqQMS/MS

70e111

Cr(III), Cu(II) Zn(II), and Pb(II)

Mineral, spring, river, and lake water (50 mL)

m-dSPE

GO@IDA (1 mg)

EDXRF

10 allergenic disperse dyes

Industrial, environmental, and wastewater (10 mL)

m-dSPE

r-GO@PSDVB (20 mg)

Melatonin, L-Tryptophan and derivates

Black sesame seeds (2 g)

m-dSPE

14 PAEs

Mineral, pond and wastewater (25 mL)

m-mdSPE

Comments

Reference

0.01e0.07 mg/L

Milk samples were deproteinized before the m-SPE extraction procedure. A comparison with GO and r-GO was carried out. The method was applied to real samples.

[59]

95e102

0.06e0.11 mg/L

Accuracy and precision of the method was tested by analyzing CRMs.

[60]

SFC-UV

89e100

1.1e15.6 mg/L

The m-dSPE procedure was assisted by sonication. A comparison with PS-DVB and two commercial sorbents (silica C18 and Oasis HLB) was carried out. The sorbent could be reused for 20 cycles.

[61]

GO@SiO2 (15 mg)

HPLC-DAD

89e115

50e100 mg/L

The m-dSPE procedure was assisted by vortex. The results obtained were confirmed by UHPLCMS/MS.

[62]

Fe3O4@rGO (60 mg)

UHPLC-QqQMS/MS

70e120

0.006e0.178 mg/L

DBP-d4 was used as surrogated.

[63]

Sildenafil citrate

Herbal products (1 g)

m-mdSPE

Fe3O4@ ND@ GO (30 mg)

HPLC-DAD

94e104

1.46 mg/L

An extraction with MeOH was previously developed. The extraction procedure was assisted by vortex. The sorbent could be reused for 10 cycles. The method was applied to real pharmaceutical herbal products samples.

[64]

8 pesticides

Cabbage (10 g)

m-mdSPE

Fe3O4@ SiO2@rGO@ VOIm+ NapSO3
(60 mg)

UHPLC-QqQMS/MS

79e96

0.01e0.15 mg/L

34 preservatives were initially selected as target analytes. An extraction with ACN/ hexane was previously developed. The extraction procedure was assisted by vortex. A comparison between four types of ILs (benzene-, naphthalene-, naphthoquinone-, and anthraquinone-ringbound imidazole structures) combined with Fe3O4@SiO2@rGO was carried out. The method was applied to real vegetable samples (cabbage, cucumber, tomato, potato, and carrot).

[65]

Continued

Table 4.3 Applications of graphene-based materials in solid-phase extraction.dcont’d Analyte

Sample (amount)

5 carbamate pesticides

Fruit juices (3 mL)

SPE mode

Sorbent (amount)

Determination technique

Recovery (%)

Detection limits

PT-SPE

r-GO (4 mg)

UHPLC-QqQMS/MS

81e125

0.0022e0.033 mg/L

Comments

Reference

A comparison with commercial sorbents (C18, PRS, and GCB) was carried out. The method was applied to real juice samples (grape, pear, and lemon juices).

[66]

m-dSPE: micro-dispersive solid-phase extraction; m-SPE: microsolid-phase extraction; ACN: acetonitrile; C18: octadecylsilane; CNT: carbon nanotube; CRM: certified reference material; DAD: diode-array detector; DBP-d4: dibutyl phthalate-3,4,5,6-d4; EDXRF: energy dispersive X-ray fluorescence spectrometry; FAAS: flame atomic absorption spectrometry; FID: flame ionization detector; GC: gas chromatography; GCB: graphitized carbon black; GO: graphene oxide; GO-NH2: amino-functionalizedegraphene oxide; HLB: hydrophilic-lipophilic balanced; HPLC: high-performance liquid chromatography; IC: ion chromatography; ICP: inductively coupled plasma; IDA: 2,20 -iminodiacetic acid; IL: ionic liquid; m-m-dSPE: magnetic-microdispersive solid-phase extraction; MeOH: methanol; MOF: metal-organic framework; MS/MS: tandem mass spectrometry; MS: mass spectrometry; ND: nanodiamond; NSAID: nonsteroidal antiinflammatory drug; OES: optical emission spectrometry; PAE: phthalic acid ester; PAH: polycyclic aromatic hydrocarbon; PEI: polyethylenimine; PFAA: perfluorinated alkyl acid; Phen: 5-amino-1,10-phenanthroline; P-p-AP: poly-(para-aminophenol); PRS: propylsulfonic acid silica; PS-DVB: poly-(styrene-co-divinylbenzene); PT: pipette tip; Q: single quadrupole; QqQ: triple quadrupole; r-GO: reduced-graphene oxide; SA: sulfonamide; S-DVB: styrene-divinylbenzene; SFC: supercritical fluid chromatography; SPE: solid-phase extraction; TFAA: trifluoroacetic acid; TPN: triazine-based polymeric network; UHPLC: ultrahigh-performance liquid chromatography; UV: ultraviolet detector; VOIm+NapSO-3: 1-vinyl-3-octylimidazole-2-naphthalene sulfonate.

Carbon-based adsorbents

105

intermediate material between graphene and GO obtained by chemical reduction using typical reducing agents such as hydrazine (N2H4) or sodium borohydride (NaBH4), among others [10,69], constitutes another alternative for graphene. Shi et al. [63] developed a method for the simultaneous determination of five carbamate pesticides residues in juice samples using pipette tip (PT)-SPE with r-GO as a sorbent for analysis by LC. Recovery values in the range 81%e125% and LODs between 0.0022 and 0.033 mg/L demonstrated the suitability of the method in which only 4 mg of adsorbent was consumed for the extraction of the target compounds. In addition, r-GO proved more effective than octadecylsiloxane-bonded silica, a silica-based propylsulfonic acid material and graphitized carbon black (GCB) for the extraction of the carbamate contaminants. In addition to the above advantages, GO and r-GO are easier to functionalize than graphene, and these modifications result in more specific interactions with target compounds resulting in more selective extraction procedures [52]. Jiang et al. [57] used r-GO in combination with gold nanoparticles (NPs) for the simultaneous m-SPE extraction of nine mycotoxins in milk. The r-GO@Au composite showed to be a more effective adsorbent than either GO or r-GO due to the gold NPs acting as nanospacers that minimize the agglomeration of the carbon-based material. The fact that small sheets of graphene, GO and r-GO can cause high back pressure and escape from the SPE cartridge limits their application in conventional SPE [51]. However, these carbon-based materials can be added directly to the sample solution providing strong interactions with target compounds in the dispersive SPE mode. Niu et al. [59] prepared GO immobilized on a silica surface by a sol-gel technique for the simultaneous clean-up and preconcentration of melatonin and structurally related compounds in black sesame seeds by m-dSPE. Only 15 mg of GO@SiO2 were required for extraction. Graphene-based materials functionalized with biomolecules [70], polymers [54e56,58,64], ionic liquids (ILs) [62], molecularly imprinted polymers (MIPs) [71e73], and other nanomaterials (e.g., metal-organic frameworks (MOFs) [53], CNTs [45] or NDs [61]) have been employed to improve extraction selectivity. Amiri et al. [53] used a hybrid nanocomposite prepared from a copperbased MOF (MOF-199) and graphene or fullerene for the m-dSPE of PAHs from water samples, Fig. 4.3. The graphene@MOF-199 nanocomposite (20 mg) offered the highest extraction efficiencies for the target compounds. The higher adsorption capacity of graphene@MOF-199 is due to its greater porosity and higher surface area than MOF-199 and fullerene@MOF-199. Graphene-based materials can also be modified with m-NPs, which allow their easy separation from the sample solution using a magnet after the extraction process, avoiding filtration or centrifugation steps [20,74]. The magnetic core is usually composed of Fe3O4 because of their easy synthesis and surface modification, low cost, and suitable stability to typical extraction conditions [10]. Li et al. [54], used a magnetic polyethyleneimine (PEI) modified r-GO composite (Fe3O4@PEI@ r-GO) for the extraction of polar NSAIDs from different water samples. The introduction of PEI as r-GO modifier not only improved its affinity for the polar compounds of interest but also increased the number of available adsorption sites. The adsorption mechanism of Fe3O4@PEI@r-GO was compared with those of

106

Solid-Phase Extraction

Figure 4.3 Scheme of the graphene@MOF-199 nanocomposite synthesis and subsequent application on the m-dSPE of PAHs from water samples. Reprinted from Amiri A, Ghaemi F, Maleki B. Hybrid nanocomposites prepared from a metalorganic framework of type MOF-199(Cu) and graphene or fullerene as sorbents for dispersive solid phase extraction of polycyclic aromatic hydrocarbons. Microchim Acta 2019;186:1e8 with permission from Springer-Verlag Wien.

Fe3O4@PEI, Fe3O4@r-GO, and Fe3O4@PEI@GO. The results demonstrated that hydrogen-bond interactions with amino groups and hydrophobic interactions with the polymer chains for the compounds of interest were responsible for its improved extraction performance compared with GO and r-GO. Graphene-based materials have also been used for on-line SPE [75,76] and in combination with other sample preparation techniques, such as DLLME [72,77].

4.2.4

Nanohorns, nanodiamonds, nanofibres, and quantum dots

In recent years, a large number of novel carbon-based materials have been evaluated as extraction sorbents in SPE, including carbon NHs (CNHs), NDs, carbon nanofibers (CNFs) and QDs as well as their modifications. CNHs are conical carbon nanostructures formed from a sp2 carbon sheet. Conceptually, single-walled CNHs (SWCNHs) are related to SWCNTs but are shaped like a horn with a narrower cone angle composed of five pentagonal rings at its apex. These structures form stable spherical aggregates of w100 nm in diameter bound by van der Waals forces [78]. CNHs are a good alternative to CNTs in many applications due to their high porosity and large surface area, high affinity for organic compounds [4], low-toxicity (absence of a toxic metal catalyst used in their synthesis compared to CNTs) and the possibility of mass production of pure material at ambient temperature. However, SWCNHs have not been as widely used as other carbon-based nanomaterials possibly owing to the difficulty of surface functionalization and limited commercial availability [78]. Table 4.4 summarizes the applications

Table 4.4 Applications of carbon-based nanohorns, nanodiamons, nanofibers, and quantum dots in solid-phase extraction. Sample (amount)

SPE mode

Sorbent (amount)

Determination technique

Recovery (%)

Detection limits

Benzophenone-3

Swimming pool water (200 mL)

m-SPE

o-SWCNHs (< 10 mg)

UHPLC-DAD

78e110

11 triazines

Water (10 mL)

m-dSPE

o-SWCNHs (0.2 mg)

GC-Q-MS

63e110

Analyte

Comments

Reference

0.16 mg/L

o-SWCNHs were immobilized on the pores of an activated borosilicate disk. The disk was coupled to a rotating metallic axle that was assembled in a drill to form a stirring extraction device.

[79]

15e100 ng/L

A dispersion of 0.2 g/L of o-SWCNHs in Milli-Q water was used. Extraction was assisted by vortex. o-SWCNHs were recovered by filtration on a 0.45 mm Nylon filter and analytes were eluted with 200 mL of MeOH. River, tap, and bottled mineral water samples were analyzed.

[80]

Nanohorns

Continued

Table 4.4 Applications of carbon-based nanohorns, nanodiamons, nanofibers, and quantum dots in solid-phase extraction.dcont’d Analyte

Sample (amount)

SPE mode

Sorbent (amount)

Determination technique

Recovery (%)

Detection limits

Edible oils (-)

OnlinemSPE

Poly-(ND@ co-C12-coTEGDMAco-TAIC) (2 mg)

HPLC-UV

90e106

3.0 mg/L

Comments

Reference

The column was connected with C18 column. The separation performance of the monolith column was evaluated for the separation of small acidic, basic, and neutral aromatic compounds. Extraction procedure was applied to commercial phytosterol mixtures, corn, corn germ, and peanut oil, and the extractive of peanut cake. Monolithic column with and without NDs were compared.

[81]

Nanodiamonds b-Sitosterol

Doxorubicin

Urine (5 mL)

m-dSPE

b-CD@NDs (2 mg)

FLD

93e94

18 mg/L

ACN was previously added to urine samples to precipitate proteins. Doxorubicin was eluted from the sorbent using a mixture of g-CDs in phosphate buffer at 0.5 g/L with 10 % of MeOH to form a more stable hosteguest inclusion complex. Fluorometric determination was compared with voltammetric and chromatographic methods.

[82]

Ziram

Water (tap and lake) (10 mL) Foodstuff (rice, cracked wheat) (1 g)

m-mdSPE

Fe3O4@NDsCOOH (25 mg)

FAAS

93e101

5.3 mg/L

Crushed foodstuffs (1 g) were previously treated with 15 mL of ACN and the final extract was submitted to the m-m-dSPE procedure. Extraction was assisted by vortex. DTC synthetic mixtures containing ziram were used to evaluate the influence of other DTCs in the determination of ziram.

[83]

Water (50 mL)

m-mdSPE

Fe3O4@ CNFs (10 mg)

GC-FID

90e101

0.008e0.030 mg/L

Desorption of analytes from sorbent was assisted by sonication. Tap, river, well, and wastewater were analyzed.

[84]

Nanofibers 3 PAHs

Continued

Table 4.4 Applications of carbon-based nanohorns, nanodiamons, nanofibers, and quantum dots in solid-phase extraction.dcont’d Analyte

Sample (amount)

SPE mode

Sorbent (amount)

Determination technique

Recovery (%)

Detection limits

4 aromatic amines

Wastewater (30 mL)

m-dSPE

CNFs@PVA (10 mg)

HPLC-UV

70e108

La(III), Ce(III), Sm(III), Eu(III), Dy(III), Y(III)

Human hair (0.5 g)

m-SPE

o-CNFs (20 mg)

ICP-MS

95e115

Comments

Reference

0.009e0.081 mg/L

CNFs@PVA solution was electrospun into fiber mats. Desorption of analytes from sorbent was assisted by sonication. Tap, river, well, and wastewater were analyzed.

[85]

0.2e1.2 ng/L

Samples were previously decomposed with concentrated nitric acid and hydrogen peroxide, and then digested in a microwave oven. 200 mL of sample extract were submitted to m-SPE procedure. Method validation was carried out using a human hair CRM.

[86]

Quantum dots Aflatoxin B1

Peanut (5 g)

SPE

C-QDs@MIP (-)

HPLC-FLD

80e91

0.118 mg/L

C-QDs were doped in MIP monolithic column. 5,7-dimethoxycoumarin as alternative template molecule instead aflatoxin B1. Peanut sample was extracted with MeOH:water (8:2, v/ v) mixture and the extract was submitted to the SPE procedure. C-QDs@MIP and C-QDs@ NIP absorption capacity were compared showing that the first one exhibited a higher affinity. Barley, maize, and barley and nutmeg traditional Chinese medicine samples were analyzed.

[87]

3 BFRs

Water (10 mL)

m-dSPE

MoS2@ C-QDs (40 mg)

HPLC-DAD

80e91

0.01e0.06 mg/L

Extraction and desorption of analytes were assisted by sonication. Desorption step was repeated twice. Single MoS2 and C-QDs, and MoS2@C-QDs were firstly compared.

[88]

Continued

Table 4.4 Applications of carbon-based nanohorns, nanodiamons, nanofibers, and quantum dots in solid-phase extraction.dcont’d Analyte BPA

Sample (amount)

SPE mode

Sorbent (amount)

Determination technique

Recovery (%)

Detection limits

Deionized water (600 mL)

m-mdSPE

Fe3O4@GQDs (50 mg)

HPLC-UV

96e105

12.3 ng/L

Comments

Reference

Adsorption efficiencies of Fe3O4, Fe3O4@graphene, and Fe3O4@G-QDs were compared. A study of the migration of BPA in mineral water samples stored in polyethylene bottles subjected to sunlight for a week was carried out. Mineral water was analyzed.

[89]

b-CD: beta-cyclodextrin; g-CD: gamma-cyclodextrin; m-dSPE: micro-dispersive solid-phase extraction; m-SPE: micro-solid-phase extraction; ACN: acetonitrile; BFR: brominated flame retardant; BPA: bisphenol A; C12: 1-dodecene; C18: octadecylsilane; CNF: carbon nanofiber; C-QD: carbon-quantum dot; CRM: certified reference material; DAD: diode-array detector; DTC: dithiocarbamate; FAAS: flame atomic absorption spectrometry; FID: flame ionization detector; FLD: fluorescence detector; GC: gas chromatography; G-QD: graphene-quantum dot; HPLC: high-performance liquid chromatography; ICP: inductively coupled plasma; m-m-dSPE: magnetic-microdispersive solid-phase extraction; MeOH: methanol; MIP: molecularly imprinted polymer; MS: mass spectrometry; ND: nanodiamond; ND-COOH: carboxylated nanodiamond; NIP: nonimprinted polymer; o-CNF: oxidized-carbon nanofiber; o-SWCNH: oxidized-single-walled carbon nanohorn; PAH: polycyclic aromatic hydrocarbon; PVA: poly-(vinyl alcohol); Q: single quadrupole; SPE: solid-phase extraction; TAIC: triallyl isocyanurate; TEGDMA: triethylene glycol dimethacrylate; UHPLC: ultrahigh-performance liquid chromatography; UV: ultraviolet detector; ziram: zinc bis(dimethyldithiocarbamate).

Carbon-based adsorbents

113

of SWCNHs and oxidized SWCNHs (o-SWCNHs) in dSPE [4]. Rold
an-Piju
an et al. [79] developed a method based on o-SWCNHs (less than 10 mg) immobilized on a stir borosilicate disk extraction device for the pre-concentration of benzophenone-3 from swimming pool water. Recovery values of 78%e110% and a LOD of 0.16 mg/ L were obtained, although the high time-consuming preparation of the extraction device and lack of automation are likely to limit its use for routine analysis. Apart from conventional SPE, other modes, such as m-dSPE, have been used with o-SWCNHs for the extraction of PAHs [80], triazines [81] and bilirubin [82] from water [80,81] and plasma samples [82], respectively. Jiménez-Soto et al. [81] employed m-dSPE for the extraction of 11 triazines in river, tap, and bottled mineral water. 10 mL of water samples were treated with 1 mL of o-SWCNHs in Milli-Q water at a concentration of 0.2 g/L and stirred for 2 min using a vortex mixer. Afterward, the sorbent was isolated by filtration on a 0.45 mm nylon filter, and the analytes desorbed with 200 mL of MeOH for GC analysis (recovery range 63%e110% and LODs of 15e100 ng/L). NDs are characterized by high hardness and chemical stability. In addition, NDs can have a high specific surface area and, although the surface of common synthetic diamonds does not have functional groups, it can be modified by coating them with polymers or by introducing different functional groups to improve the selectivity [4,83]. Currently, applications of NDs for SPE are scarce, perhaps due to their high cost in comparison with other sorbents [83]. As shown in Table 4.4, NDs have been combined with other materials, such as GO [61] or MoS2 [84], functionalized by oxidation [85] or coated with polyarginine [86], single/double-arm amide-thiourea ligands [87], b-cyclodextrin (b-CD) [88], bacteria [89] and polymeric mixtures [90] to improve their extraction performance for a wider range of compounds. Cui et al. [90] prepared a monolithic column with NDs modified in multiple steps by a triethylene glycol dimethacrylate (TEGDMA) and triallyl isocyanurate (TAIC) polymer, poly(ND@co-C12-co-TEGDMA-co-TAIC) for the online (m-SPE) of b-sitosterol in edible oil (recovery range 90%e106% and LOD of 3 mg/L). The extraction procedure was also applied to other vegetable samples, such as corn, corn germ, peanut oil, and peanut cake. The polymer monolith containing NDs provided better extraction performance than the naked polymer monolith. Bacillus altitudinis immobilized on NDs was used for the pre-concentration of metals ions in water and food samples (apple juice, strawberry juice, energy drink, meat, chicken, flour, honey, milk, olive, white cheese, corn, tomato, potato, and black tea samples) by SPE [89]. The modification of the ND surface with the specific bacteria provided low LODs in the range 0.016e0.071 mg/L. Most applications employing NDs used the dSPE mode [84e88]. The NDs were generally functionalized [85e88] or used in combination with other sorbents [84] improving the selectivity and versatility of the materials. They were used for the extraction of drugs [88], oligosaccharides [86] membrane proteins [85], and metal ions [84,87] in diluted solutions of artificial cerebrospinal fluid [86], methylococcus capsulatus and E. coli cells [84,85], water [87] and urine samples [88]. Yilmaz et al. [91] used magnetic carboxylated-NDs (NDs-COOH) for the extraction of ziram fungicide (zinc bis-(dimethyldithiocarbamate) complex) from water,

114

Solid-Phase Extraction

foodstuffs, and synthetic mixtures using a vortex-assisted m-m-dSPE procedure with 10 mL of the sample solution and 25 mg of Fe3O4@NDs-COOH. The solution was vortexed for 2 min, the sorbent isolated by a magnet, and the ziram eluted with 1 mL of 0.25 M HNO3 in acetone. The total extraction time was less than 10 min per sample, recovery values between 93% and 101% and a LOD of 5.3 mg/L. In another application, Yilmaz et al. [61] used a magnetic sorbent based on NDs and GO for the extraction of sildenafil from alleged herbal aphrodisiacs. In this case, the aim of NDs was to avoid the problems of aggregation and restacking of GO nanosheets by their insertion between the GO sheets. CNFs lack a hollow cavity, and their diameters are generally larger than those of CNTs. They are solid carbon fibers a few microns long with diameters less than 100 nm. An attraction for SPE is their uniform mesoporous structure with a high specific surface area, up to 1877 m2/g, among the highest values recorded for a nanostructured material [4]. CNFs are expected to have a high adsorption capacity. However, untreated CNFs are nonpolar and have a low affinity for the extraction of polar compounds. Their surface can be treated to form certain functional groups that favor the establishment of specific interactions [92]. Generally, CNFs have been used alone as sorbents in SPE [93e97] and in a few cases after surface functionalization [92,98,99] or in combination with other sorbents, such as m-NPs [100,101] or graphene [101]. Chen et al. [92] extracted trace rare earth elements (including La(III), Ce(III), Sm(III), Eu(III), Dy(III), and Y(III)) in certified reference material (CRM) of human hair using oxidized CNFs (o-CNFs) as a sorbent in m-SPE (recovery range 95%e115% and LODs around 0.2e1.2 ng/L). The adsorption capacity of o-CNFs was compared with MWCNTs and activated carbon under the same conditions, demonstrating a higher adsorption capacity for the o-CNFs due to the formation of polar groups on its surface. In addition to the SPE of metal ions [92,94,95,97], CNFs were also used for the extraction of triazine herbicides [93], fungicides [96], and explosive compounds [99] in various samples, such as soil [93], water [94,99], CRMs for tea leaves [95], human hair [92] and mussels [95], apple juice [96], lemon [96] and cucumber [96]. Although CNFs have been used mainly as sorbents for conventional SPE, they have also been used in the m-dSPE [98] and m-m-dSPE [100] formats for the extraction of aromatic amines and PAHs, respectively, from water samples. QDs first identified in 2004 [102] during the purification of SWCNTs by electrophoresis, are zero-dimensional, chemically inert nanomaterial with high fluorescence activity, highly water-soluble, small particle sized (less than 10 nm), have a high surface-to-volume ratio, negligible toxicity, and excellent photostability. In addition, QDs are highly biocompatible and environmentally friendly. Also, these NPs represent a good alternative to conventional inorganic semiconductor QDs [103,104]. In general terms, QDs can be classified into two groups that differ in some structural features: carbon-QDs (C-QDs) and graphene-QDs (G-QDs). While C-QDs are quasi-spherical NPs with diameters lower than 10 nm, G-QDs consist of graphene nanosheets of less than 10 nm thickness with single, double, or multiple layers [103]. Since most of the synthesized QDs are NPs that contain carboxyl, hydroxyl, carbonyl, epoxide, and amino functional groups, among others, on its surface, they have a high adsorption capacity due to the presence and number of active sites [103,105]. Despite their theoretical

Carbon-based adsorbents

115

advantages as extraction sorbents, they have scarcely been exploited in this role [106] compared with potential applications as sensors or biosensors [104]. Table 4.4 summarizes some recent applications for SPE. Liang et al. [107] prepared a MIP-based monolithic column doped with C-QDs for the selective extraction of aflatoxin B1 from food samples (peanut, barley, maize, nutmeg) and traditional herbal medicine. The C-QDs in the C-QDs@MIP monolith increased the adsorption capacity of the column. QDs have been applied as sorbents either as raw materials [108] or functionalized [106,109,110] and in combination with other types of sorbents [105e107,109e115]. Razmi et al. [108] provided the only example of the use of single G-QDs, in the form of eggshell, for the extraction of PAHs from water. The high hydrophilicity resulting from the presence of surface functional groups increased the dispersion capacity of QDs in aqueous solution but hindered their separation from the medium, which limits their application as an adsorbent in dSPE [105,115]. Therefore, in general, the QDs have been supported on other materials for applications in m-dSPE [105,106,115] or combined with m-NPs for m-dSPE [113] and m-m-dSPE [109,111,112,114] formats. Dong et al. [105] combined the advantages of C-QDs with those of MoS2 for the m-dSPE of three brominated flame retardants (BFRs) from water samples. The intrinsic properties of C-QDs added to the layered structure of MoS2 provide favorable conditions for the extraction with the recovery values of 80%e91% and LODs of 0.01e0.06 mg/L for the brominated flame retardants. To demonstrate the extraction capacity of the prepared composite, single MoS2, and C-QDs, and MoS2@ C-QDs were compared, demonstrating the synergy of interactions for the MoS2@ C-QDs sorbent. Although for conventional SPE, QDs have only been used for the extraction of PAHs [108], pesticides [110] and aflatoxins [107], their potential for the extraction of other organic compounds, such as bisphenol A (BPA) [111], BFRs [105], and metal ions [106,109,112,114,115] by m-dSPE or m-m-dSPE has received attention. Mohammad-Rezaei et al. [111] synthesized a magnetic nanocomposite composed of m-NPs and G-QDs by the hydrothermal method. The Fe3O4@G-QDs adsorbent was used for the preconcentration of BPA in drinking water samples (recovery range 96%e105% and LOD of 12.3 ng/L). Fig. 4.4 [116] illustrates the synthesis of G-QDs and graphene (or GO) by pyrolysis of citric acid. The G-QDs consist of graphene sheets less than 10 nm thick with a lateral size of 100 nm. The transformation of the graphene sheets into the G-QDs considerably increases the extraction capacity of the sorbent due to the high surface-to-volume ratio and high adsorption capacity for aromatic compounds due to p-p interactions. QDs have high water solubility and can be used to improve the suitability of other sorbents for extraction from aqueous solution. Yang et al. [117] used a C-QDs-based composite to enhance the dispersion and hydrophilicity of an IL in water and milk samples. The nanomaterial (a ferrofluid) was formed between m-NPs coated with oleic acid, C-QDs, and 1-octyl-3-methylimidazolium hexafluorophosphate ([C8MIM]PF6) IL. The C-QDs not only enhanced the dispersibility of the sorbent material but also improved the reproducibility and precision of the method for the extraction of four phenolic compounds.

116

Solid-Phase Extraction

Figure 4.4 Diagram for the synthesis of GQDs and GO. The black dots in the GO represent oxygen atoms. Reprinted from Dong Y, Shao J, Chen C, Li H, Wang R, Chi Y, Lin X, Chen G. Blue luminescent graphene quantum dots and graphene oxide prepared by tuning the carbonization degree of citric acid. Carbon 2012;50:4738e4743 with permission from Elsevier.

4.3

Conclusions

The search for suitable materials as adsorbents to be used in SPE is an important research area in material science and sample preparation. Among the numerous materials evaluated for SPE carbon-based nanomaterials is an important group with several advantages: favorable mechanical stability, high surface to volume ratio, and the capability to modify selectivity by surface modification processes for the extraction of organic and inorganic compounds from a variety of sample types. Undoubtedly, CNTs and graphene have been the most widely used carbon-based nanomaterials for SPE due to their ease of modification and a large number of commercial products available. However, other materials such as fullerenes, NHs, NDs, CNFs, and C-QDs are less well established but have demonstrated a potential for future developments.

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List of abbreviations b-CD g-CD [C8MIM]PF6 m-dSPE m-ECD m-SPE 8-iso-PGF2a-d4 AAS ACN BFR BP BPA C12 C18 CE ChCl CNF CNH CNT C-QD CRM DAD DBP-d4 DCM DLLME dSPE DTC DWCNT ECD EDXRF FAAS FID FLD GC GCB GO GO-NH2 G-QD HLB HPLC IC ICP IDA IL IS LC

Beta-cyclodextrin Gamma-cyclodextrin 1-octyl-3-methylimidazolium hexafluorophosphate Micro-dispersive solid-phase extraction Micro-electron capture detector Micro-solid-phase extraction 8-iso-prostaglandin-F2a-d4 Atomic absorption spectrometry Acetonitrile Brominated flame retardant Buckypaper Bisphenol A 1-dodecene Octadecylsilane Capillary electrophoresis Choline chloride Carbon nanofiber Carbon nanohorn Carbon nanotube Carbon-quantum dot Certified reference material Diode-array detector Dibutyl phthalate-3,4,5,6-d4 Dichloromethane Dispersive liquid-liquid microextraction Dispersive solid-phase extraction Dithiocarbamate Double-walled carbon nanotube Electron capture detector Energy dispersive X-ray fluorescence spectrometry Flame atomic absorption spectrometry Flame ionization detector Fluorescence detector Gas chromatography Graphitized carbon black Graphene oxide Amino-functionalized-graphene oxide Graphene-quantum dot Hydrophilic-lipophilic balanced High performance liquid chromatography Ion chromatography Inductively coupled plasma 2,20 -iminodiacetic acid Ionic liquid Internal standard Liquid chromatography

118

LDH LOD LVI m-m-dSPE MALDI m-dSPE MeOH MIP m-MWCNT m-NPs MOF MS MS/MS MWCNT MWCNT-COOH MWCNT-NH2 MWCNT-OH ND ND-COOH NH NIP NP NSAID o-CNF OCP OES o-MWCNT o-SWCNH PAE PAH PCB PDA PEI PFAA Phen P-p-AP PRS PS-DVB PT PTFE PVA Q QD QqQ QTrap r-GO RP SA S-DVB

Solid-Phase Extraction

Layered double hydroxide Limit of detection Large-volume injection Magnetic micro-dispersive solid-phase extraction Matrix-assisted laser desorption/ionization Magnetic dispersive solid-phase extraction Methanol Molecularly imprinted polymer Magnetic multi-walled carbon nanotube Magnetic nanoparticles Metal-organic framework Mass spectrometry Tandem mass spectrometry Multi-walled carbon nanotube Carboxylated multi-walled carbon nanotube Amine functionalized-multi-walled carbon nanotube Hydroxylated multi-walled carbon nanotube Nanodiamond Carboxylated-nanodiamond Nanohorn Nonimprinted polymer Nanoparticle Nonsteroidal antiinflammatory drug Oxidized carbon nanofiber Organochlorine pesticide Optical emission spectrometry Oxidized multi-walled carbon nanotube Oxidized single-walled carbon nanohorn Phthalic acid ester Polycyclic aromatic hydrocarbon Polychlorinated biphenyl Photodiode array detector Polyethylenimine Perfluorinated alkyl acid 5-amino-1,10-phenanthroline Poly-(para-aminophenol) Propylsulfonic acid silica Poly-(styrene-co-divinylbenzene) Pipette tip Polytetrafluoroethylene Poly-(vinyl alcohol) Single quadrupole Quantum dot Triple quadrupole Triple quadrupole linear ion trap Reduced-graphene oxide Reverse phase Sulfonamide Styrene-divinylbenzene

Carbon-based adsorbents

SFC SFOD SPE SWCNH SWCNT TAIC TEGDMA TFAA ToF TPN UHPLC UV VOImDNapSO3L VWD Wt Ziram

119

Supercritical fluid chromatography Solidified floating organic droplets Solid-phase extraction Single-walled carbon nanohorn Single-walled carbon nanotube Triallyl isocyanurate Triethylene glycol dimethacrylate Trifluoroacetic acid Time-of-flight Triazine-based polymeric network Ultra-high performance liquid chromatography Ultraviolet detector 1-vinyl-3-octylimidazole-2-naphthalene sulfonate Variable-wavelength detector Weight Zinc bis-(dimethyldithiocarbamate)

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arcel M. Cyclodextrin-modified nanodiamond for the sensitive fluorometric determination of doxorubicin in urine based on its differential affinity towards b/g-cyclodextrins. Microchim Acta 2018;185:115. [89] Ozdemir S, Kilinc E, Celik KS, Okumus V, Soylak M. Simultaneous preconcentrations of Co2þ, Cr6þ, Hg2þ and Pb2þ ions by Bacillus altitudinis immobilized nanodiamond prior to their determinations in food samples by ICP-OES. Food Chem 2017;215:447e53. [90] Cui B, Liu H, Yu H, Pang X, Yan H, Bai L. Monolithic material prepared with nanodiamond as monomer for the enrichment of b-sitosterol in edible oil. Food Anal Methods 2019;12:697e704. [91] Yılmaz E, Soylak M. Preparation and characterization of magnetic carboxylated nanodiamonds for vortex-assisted magnetic solid-phase extraction of ziram in food and water samples. Talanta 2016;158:152e8. [92] Chen S, Xiao M, Lu D, Zhan X. Use of a microcolumn packed with modified carbon nanofibers coupled with inductively coupled plasma mass spectrometry for simultaneous on-line preconcentration and determination of trace rare earth elements in biological samples. Rapid Commun Mass Spectrom 2007;21:2524e8. [93] Boonjob W, Mir
o M, Segundo MA, Cerda. Flow-through dispersed carbon nanofiberbased microsolid-phase extraction coupled to liquid chromatography for automatic determination of trace levels of priority environmental pollutants. Anal Chem 2011;83: 5237e44. [94] Chen S, Zhan X, Lu D, Liu C, Zhu L. Speciation analysis of inorganic arsenic in natural water by carbon nanofibers separation and inductively coupled plasma mass spectrometry determination. Anal Chim Acta 2009;634:192e6. [95] Chen S, Xiao M, Lu D, Zhan X. Carbon nanofibers as solid-phase extraction adsorbent for the preconcentration of trace rare earth elements and their determination by inductively coupled plasma mass spectrometry. Anal Lett 2007;40:2105e15. [96] Wang L, Zhang M, Zhang D, Zhan L. New approach for the simultaneous determination fungicide residues in food samples by using carbon nanofiber packed microcolumn coupled with HPLC. Food Control 2016;60:1e6. [97] Chen S, Xiao M, Lu D, Wang Z. The use of carbon nanofibers microcolumn preconcentration for inductively coupled plasma mass spectrometry determination of Mn, Co and Ni. Spectrochim Acta Part B 2007;62:1216e21. [98] Vadukumpully S, Basheer C, Jeng CS, Valiyaveettil S. Carbon nanofibers extracted from soot as a sorbent for the determination of aromatic amines from wastewater effluent samples. J Chormatogr A 2011;1218:3581e7.

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Solid-Phase Extraction

[99] Tavengwa NT, Hintsho N, Durbach S, Weiersbye I, Cukrowska E, Chimuka L. Extraction of explosive compounds from aqueous solutions by solid phase extraction using b-cyclodextrin functionalized carbon nanofibers as sorbents. J Environ Chem Eng 2016; 4:2450e7. [100] Sarafraz-Yazdi A, Rokhian T, Amiri A, Ghaemi F. Carbon nanofibers decorated with magnetic nanoparticles as a new sorbent for the magnetic solid phase extraction of selected polycyclic aromatic hydrocarbons from water samples. New J Chem 2015;39: 5621e7. [101] Rezvani-Eivari M, Amiri A, Baghayeri M, Ghaemi F. Magnetized graphene layers synthesized on the carbon nanofibers as novel adsorbent for the extraction of polycyclic aromatic hydrocarbons from environmental water samples. J Chromatogr A 2016;1465: 1e8. [102] Xu X, Ray R, Gu Y, Ploehn HJ, Gearheart L, Raker K, Scrivens WA. Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments. J Am Chem Soc 2004;126:12736e7. [103] Benítez-Martínez S, Valc
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Restricted access media

5

Maria Eugênia C. Queiroz, Israel D. Souza Departamento de Química, Faculdade de Filosofia Ciências e Letras de Ribeir~ao Preto, Universidade de S~ao Paulo, Ribeir~ao Preto, SP, Brasil

5.1

Introduction

Trace analysis of drugs and their metabolites or biomarkers in biological matrixes is a challenge that always has to be faced during pharmaceutical, clinical, omics, and toxicological studies. Considering the physicochemical properties of these analytes, liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) has become the most sensitive and selective analytical technique for their analysis in these complex samples. Conversely, direct injection of biological samples into conventional LC-MS/ MS systems is inappropriate. Endogenous macromolecules, primarily proteins, can adsorb onto the column, leading to back-pressure build-up, modifying retention times, decreasing column efficiency, and suppressing ions during electrospray ionization mass spectrometry. In this context, a sample preparation step preceding the chromatographic separation is essential to isolate the analytes and to remove the endogenous interferents. Recent trends in sample preparation techniques include miniaturization, automation, and development of selective extraction sorbents. In this scenario, restricted access media (RAM) sorbents integrated with an LC system allows the direct highthroughput online analysis of biological samples.

5.2

Restricted access media

Desilets et al. introduced the term restricted access media (RAM) to describe a group of phases that enable direct and repetitive injection of untreated biological samples into a reversed-phase LC system by limiting the accessibility of macromolecules (sample matrix) to the adsorption sites of the porous supports [1]. Access is restricted on the basis of size; size exclusion combined with conventional hydrophobic or ion exchange interactions allows small molecules to pass while restricting the access of macromolecules. Therefore, macromolecules such as proteins from biological matrixes are excluded (by physical or chemical means, or a combination of both) in the void volume, whereas small hydrophobic analytes are retained [1e4]. The excluded macromolecules only interact with the outer surface of the support coated with hydrophilic groups, minimizing matrix protein adsorption. Considering the protein exclusion mechanism (or the barrier nature), RAM can be classified into (a) physical diffusion barrier: macromolecules are excluded due to pore diameter, Solid-Phase Extraction. https://doi.org/10.1016/B978-0-12-816906-3.00005-4 Copyright © 2020 Elsevier Inc. All rights reserved.

130

Solid-Phase Extraction

Figure 5.1 Schematic representation of (A) alkyl-diol-silica, (B) protein-coated silica and (C) RAMIP material.

and (b) chemical diffusion barrier: macromolecules are excluded by a network covering the outer surface of the particle, which consists of synthetic/natural polymers or protein covalently bound or adsorbed at the outer surface of silica [1e4], Fig. 5.1. Apart from this classification, these phases have been characterized according to the sorbent surface structure. Bimodal phases consist of a hydrophilic exterior and hydrophobic internal surfaces, while unimodal phases comprise inner and outer surfaces displaying the same properties [1,4]. A further class of RAM is (c) RAMIPs. These combine an efficient ability at removing macromolecules with a higher selectivity based on molecular recognition of the analyte, Fig. 5.1.

5.2.1 5.2.1.1

RAM with a physical barrier Alkyl-diol-silica material

The alkyl-diol-silica (ADS) sorbent is the most common RAM material. Hydrophilic electroneutral groups (glycerylpropyl; i.e., diol moieties) bound at the outer surface of the particles exclude macromolecules through a physical barrier (size-exclusion process, pore size ¼ 6 nm), avoiding macromolecule adsorption onto the support, Fig. 5.1A. The inner surface of the porous particles (reversed phase or strong cation-exchanger, SCX) adsorbs the target compounds [1e4]. Table 5.1 summarizes different approaches for online extraction systems with LiChropher RP8-ADS [5e9] and RP18-ADS [10e12] cartridges, which are suitable for direct analysis of biological fluids with minimal matrix effects. Cation-exchange RAMs have been synthesized to extract basic compounds for pharmaceutical analysis [13]. A simple system that combines online diol silica SCX-RAM extraction with two-dimensional RP  RP LC-MS/MS was used for the determination of peptides in human serum [14].

Table 5.1 LC methods to analyze compounds by direct injection of biological or environmental matrixes into RAM columns.

Analytes

Matrix

RAM sorbent

Ext. tech.

Sample volume (mL)

Analytical Column

LC system

Linear range (ng/mL)

Ref.

Ext. conditions Sorption

Elution

RAM with a chemical barrier a1-adrenergic receptor antagonists

Rat feces

Capcell Pak MF (C8-protein)

CS

e

Phosphate buffer pH 7.4/ACN (92:8, v/v)

Acetate buffer pH 5.3/ MeOH/ACN (22:45:33, v/v/v)

Chiralpak IA column (250  4.6 mm, 5 mm)

LC-FD

22.5e15,000

[17]

Olanzapine, fluoxetine and norfluoxetine

Plasma

Capcell MF Ph-1 (MM)

CS

200

10 mM ammonium formate/ACN (99:1, v/v)

10 mM ammonium formate with 0.01% formic acid/ACN (65:35 v/v)

Hypersil Gold C18 column (150  4.6 mm, 5 mm)

LC-MS/ MS

0.5e100

[18]

Sterols

Serum

BioTrap 500C18 (MM)

CS

300

Water

Water/2propanol (60: 40, v/v)

XTerras MS C18 column (4.6  100 mm, 3.5 mm)

LC-MS/ MS

17e2700

[19]

Albendazole metabolites

In-vitro microsomal culture

Phenyl-BSA

2D

350

Water

Water/MeOH (50:50, v/v)

Amylose tris (3,5dimethylphenyl carbamate) (150  4.6 mm, 7 mm)

LC-UVvis

e

[21]

Statins

Plasma

C18-BSA

CS

500

100 mM potassium phosphate pH 2.5/ACN (60: 40, v/v)

100 mM phosphate buffer pH 2.5/ACN (40: 60, v/v)

Lichrocart 125e4 Purospher StarC8 (125  4.0 mm, 5 mm)

LC-PDA

125e876

[22]

Fluoroquinolones

Water

BSA-octyl-silica

SC

2000

(Water with 0.1% formic acid)/ ACN (90:10, v/v)

0.1% formic acid/ACN (10:90, v/v)

e

LC-MS/ MS

20e800

[23]

Continued

Table 5.1 LC methods to analyze compounds by direct injection of biological or environmental matrixes into RAM columns.dcont’d

Ext. tech.

Sample volume (mL)

Analytes

Matrix

RAM sorbent

Antihypertensives

Serum

CNTs-BSA

CS

Anticonvulsants

Plasma

CNTs-BSA

Tetracyclines

Milk

Enrofloxacin andgatifloxacin

Milk

Ext. conditions Analytical Column

LC system

Linear range (ng/mL)

Ref.

Sorption

Elution

100

10 mM ammonium formate

10 mM ammonium formate pH 5.0)/MeOH (90:10, v/v)

Shim-pack XRODS C18 (150  4.6 mm, 5 mm)

LC-MS/ MS

200e500

[26]

CS

100

Water

0.01 mol/L phosphate buffer pH 6.0/ACN/ MeOH (55: 25:20; v/v/v)

C18 (250  4.6 mm, 5 mm)

LC-UV

2000e40,000

[27]

CNTs-BSA

CS

500

0.10 mM oxalic acid/ACN (80: 20, v/v)

10 mM oxalic acid/ACN (70:30, v/v)

Shim-pack XRODS C18 (150  4.6 mm, 5 mm)

LCDAD

50,000e200,000

[28]

Sil-g-p(St/DVB)Diol/BSA

CS

1000

100 mM phosphate buffer pH 7.0/ ACN/trifluoro acetic acid/ isopropanol (90:10:1:1, v/v/v)

Water/ACN with 0.1% formic acid (95/5, v/v)

Luna (150  4.6 mm, 5 mm)

LCDAD

10e300

[29]

Sil-g-p(SPM/ EDMA)-Diol

CS

1000

5 mM ammonium acetate pH 3.0/ ACN (70/30, v/v)

10 mM ammonium acetate pH 8.0/ACN (15/ 85, v/v)

HILIC Luna (150  4.60 mm, 5 mm)

LC-PDA

5e200,000

[15]

RAM with a physical barrier Melamine and cyromazine

Milk

Sulfonamides

Honey

Hybrid silica monolith (C3-ADS)

2D

4.2 g

Water/ACN (75: 25, v/v)

100 mM phosphate buffer/ACN (15:85, v/v)

Venusil XBP C18 (250  4.6 mm, 5 mm)

LC-UV

0.05e0.2

Mercapturic acids derived

Urine

LiChrospher (RP8-ADS)

CS

200

0.05% formic acid

0.05% formic acid/ACN (50:50, v/v)

Zorbax Eclipse XDB C18 (150  2.1 mm, 3.5 mm)

LC-MS/ MS

Mercapturic acids

Urine

LiChrospher (RP8-ADS)

CS

100

Water with 0.1% formic acid

0.1% formic acid/ACN 0.1% formic acid (88:12, v/v)

Luna C8 (150  4.6 mm, 3 mm)

LC-MS/ MS

0.03e150

[6]

1-vinyl-2pyrrolidonemercapturic acid

Urine

LiChrospher (RP8-ADS)

CS

100

Water pH 2.5/ ACN with 0.1% formic acid (88:12, v/v)

Water pH 2.5/ ACN with 0.1% formic acid (75:25, v/v)

Luna 3u C8(2) 100A (150  4.6 mm, 3 mm)

LC-MS/ MS

500e500,000

[7]

Cortisol and cortisone

Hair

LiChrospher (RP8-ADS)

CS

50 mg

0.02% ammonium hydroxide/ MeOH (90:10, v/v)

Water/MeOH (20:80, v/v)

Poroshell 120 ECC18 (50  4.6 mm, 2.7 mm)

LC-MS/ MS/ MS

2e200 pg/mg

[8]

Psychotropics

Plasma

LiChrospher (RP8-ADS)

CS

200

Water

4 mM ammonium acetate with 0.1% formic acid/ACN (63:37, v/v)

Kinetex C18 (100  2.1 mm, 1.7 mm)

LC-MS/ MS

0.025e1.25

[9]

Atenolol and propranolol

Plasma

LiChrospher (RP18-ADS)

multisyringe flow injection analysis

200

Water/ACN (98: 2, v/v)

ACN

AV-5-MS (30 m  0.25 mm, 0.25 mm film thickness)

GC-MS

6e1000

[10]

0.10e20.0

[16]

[5]

Continued

Table 5.1 LC methods to analyze compounds by direct injection of biological or environmental matrixes into RAM columns.dcont’d

Ext. tech.

Sample volume (mL)

LiChrospher (RP18-ADS)

2D

Urine

LiChrospher (RP18-ADS)

Peptides

Serum

Xenobiotics and endogenous metabolites

Analytes

Matrix

Alprenolol and propranolol

Blood

Mercapturic acids of naphthalene

RAM sorbent

Ext. conditions Analytical Column

LC system

Linear range (ng/mL)

Ref.

Sorption

Elution

100

Water/ACN (98:2, v/v)

TFA solution with 0.1% triethylamine/ ACN (74:26, v/v)

Luna PFP2 (150  4.6 mm, 3 mm)

LC-FD

5e200

[11]

2D

1000

1% acetic acid

100 mM ammonium acetate buffer pH 4.5/ MeOH (60: 40, v/v)

KinetexBiphenyl Column (100  2.1 mm, 2.6 mm)

LC-MS/ MS

800e20,000

[12]

Lichroprep (SO3-Diol)

2D

400

20 mM ammonium acetate pH3.0

1000 mM ammonium acetate pH 3.0

Kromasil-ODS 300 Å (150  4.6 mm, 5 mm)

LC-MS/ MS

e

[14]

Urine

MSpak 4A (styrene-PVA)

2D

10

Water

2.5 mM formic acid/ACN (10:90, v/v)

ZIC-HILIC and Luna PFP (150  4.60 mm, 3.5 mm)

LC-MS/ MS

Vanillin and metabolites

Plasma

Strata-X (MM)

CS

100

Water/ACN (90:10, v/v)

0.2% formic acid/ACN (10:90, v/v)

Kinetex coreeshell C18 column (50  4.6 mm, 2.6 mm)

LC-MS/ MS

5e1000

[52]

Lipid mediators

Skeletal muscles

SUPELCOSIL LC-HISEP (MM)

2D

100 mg

10 mM ammonium acetate

0.1% formic acid

Ultra C8 (150  2.1 mm, 3 mm)

LC-MS/ MS

0.031e32

[53]

2e10

[51]

RAMIPs 2-Arylpropionic acid derivatives

Serum

RAMIP (hydroxyl)

CS

20

20 mM phosphoric acid/ acetonitrile pH 2.2 (78:22, v/v)

20 mM sodium phosphate buffer pH 7.3/ACN (75:25, v/v)

Cosmosil 5C18MS-II (150  4.6 mm, 5 mm)

LC-UV

200e50,000

[31]

b-blockers

Plasma

RAMIP (hydroxyl)

CS

0.05

20 mM sodium phosphate buffer pH 5.8/ ACN (80:20, v/v)

20 mM sodium phosphate buffer pH 3.7/ACN (78:22, v/v)

ULTRON ESPhCD (150  6 mm, 5 mm)

LC-UV

12.5e250

[32]

Phenobarbital, amobarbital, and phenytoin

River water

RAMIP (hydroxyl)

CS

50,000

2 mM ammonium acetate

2 mM ammonium acetate/ACN (60:40, v/v)

Cosmosil 5C18MS-II (150  4.6 mm, 5 mm)

LC-MS

0.5e50

[36]

Methylthiotriazine herbicides

River water

RAMIP (hydroxyl)

CS

100,000

Water

50 mM potassium phosphate buffer pH 7.0/ACN (62:38, v/v)

Cosmosil 5C18MS-II (150  4.6 mm, 5 mm)

LC-UV

0.05e0.5

[37]

Sulfonyl urea residue

Soil

RAMIP (hydroxyl)

CS

10 g

50 mM phosphate buffer pH 5.0/ ACN (90:10, v/v)

50 mM phosphate buffer pH 3.3/ACN (60:40, v/v)

Gemini C8 (150  4.6 mm, 5 mm)

LC-UVvis

10e5000

[38]

Clenbuterol

Serum

RAMIP (hydroxyl)

CS

1000

10 mM phosphate buffer pH 7.0/ ACN (80:20, v/v)

10 mM phosphate buffer pH 3.0/ACN (75:25, v/v)

Kromasil C18 column (250  4.6 mm, 5 mm)

LC- UVvis

2e1000

[39]

Sulfonamides

Milk

RAMIP (hydroxyl)

CS

100

100 mM phosphate buffer pH 6.0/ MeOH (95:5, v/v)

100 mM phosphate buffer pH 7.0/ACN (83:17, v/v)

Phenomenex C18 (250  4.6 mm, 5 mm)

LC-UV

2e400

[45]

Continued

Table 5.1 LC methods to analyze compounds by direct injection of biological or environmental matrixes into RAM columns.dcont’d

Ext. tech.

Sample volume (mL)

Ext. conditions Sorption

Elution

Analytical Column

LC system

Linear range (ng/mL)

Ref.

Analytes

Matrix

RAM sorbent

Oxacillin and cloxacillin

Urine and plasma

RAMIP (hydroxyl)

CS

100

Water

25 mM phosphate buffer pH 4.5/ACN (74:26, v/v)

C18 diomonsil column (250  4.6 mm, 5 mm)

LC-UV

1000e60,000

[46]

Chlorpromazine

Plasma

RAMIP (hydroxyl/ BSA)

CS

100

Water

20 mM acetate pH 4.1/ACN 55:45 (v/v)

Phenomenex C18 (250  4.6 mm, 5 mm)

LC-UVvis

30,000e350,000

[47]

Serotonin reuptake inhibitors

Plasma

RAMIP (hydroxyl/ BSA)

CS

200

e

20 mM acetate buffer pH 4.5/MeOH/ ACN (60:3: 37 v/v/v)

Phenomenex C18 (250  4.6 mm, 5 mm)

LC-UVvis

20e500

[48]

Ivermectin

Meat

RAMIP (hydroxyl/ BSA)

CS

500 mg

e

ACN/MeOH/ water, (60:30: 10 v/v/v)

Phenomenex C18 (250  4.6 mm, 5 mm)

LC-UVvis

50e500 mg kg

[49]

Oxprenolol

e

RAMIP (hydroxyl/ BSA)

CS

100

10 mM ammonium formate pH 3.7/MeOH (95:5, v/v)

Shim-PackXRODS C18 (100  3 mm, 2.2 mm)

LC-MS

e

[50]

Tricyclic antidepressants

Plasma

BSA-MIP

SC

1000

Water

0.01% acetic acid/ACN (30:70, v/v)

e

-MS

15,000e500,000

[54]

Phenobarbital

Serum

RAMIP (glycopolymer)

CS

20

Water

MeOH

Comatex-AB C18 column (250  4.6 mm, 5 mm)

LC-UV

e

[55]

ACN, acetonitrile; ADS, alkyl-diol silica; CS, column-switching; MM, mixed mode material; MeOH, methanol; SC, single mode column; TFA, trifluoro acetic acid.

1

Restricted access media

5.2.1.2

137

SCX-RAM synthesized by surface-initiated atom transfer radical polymerization

A new SCX-RAM has been developed by sequentially grafting different polymers on silica via the surface-initiated atom transfer radical polymerization (ATRP) technique. During the synthesis, poly(3-sulfopropyl methacrylate-co-ethylene dimethacrylate) [p(SPM/EDMA)] was grafted onto the silica surface initially through the ATRP “grafting form”. Then, a hydrophilic chemical barrier, poly(glycerol mono-methacrylate) [p(GMMA)], was established by using ATRP to graft glycidyl methacrylate (GMA) on the p(SPM/EDMA)-grafted silica, which was followed by hydrolysis of the epoxy groups. The resulting Sil-g-p(SPM/EDMA)-g-pGMMA can exclude proteins during HPLC analysis [15].

5.2.1.3

Monolithic phases

Monolithic phases, which are porous rod structures characterized by mesopores and macropores, are also suitable for direct injection of biological fluids. Due to the high permeability of monolithic phases, extractions can be performed at a high flow rate without generating high backpressure. A novel RAM hybrid monolithic column (RAM-HMC) prepared by in situ synthesis (sol-gel procedure) in a stainless-steel column was developed to clean-up honey samples online and to preconcentrate the sulfonamide residues. This RAM-HMC can effectively exclude macromolecules and enrich the samples containing the target analytes. A hydrophobic monolithic column (HMC) was prepared by the acid catalyzed reaction of methyltrimethoxysilane with tetraethoxysilane. Hydrophilic structures originated on the through-pore surface of the HMC by grafting of 3-(2,3-epoxypropoxy)propyltrimethoxysilane. The optimized synthetic conditions provided a uniform microchannel and a stable skeletal structure [16]. Monolithic columns developed by the in-situ sol-gel polymerization technology do not require frits at column extremities, which are often the main source of endogenous material adsorption.

5.2.2 5.2.2.1

RAM with a chemical barrier Mixed-functional material

A mixed-functional extraction RAM sorbent is commercialized under the trade name Capcell Pak MF. Both the internal and external surfaces comprise a mixture of hydrophilic poly(oxyethylene) and hydrophobic styrene groups grafted on a porous silicone polymer-coated silica substrate. The long poly(oxyethylene) chains limit access of macromolecules to retentive surfaces. Therefore, matrix components only interact with the hydrophilic, nonadsorptive polymer network, and elute in the void volume [2,3]. By using a Capcell Pak MF C8 column in the first dimension, an online columnswitching chiral high-performance liquid chromatography (HPLC) method was developed to determine naftopidil and the enantiomers of its O-desmethyl metabolites

138

Solid-Phase Extraction

simultaneously in rat feces. Direct and multiple injections of the supernatant from rat feces homogenate through the column-switching system was possible [17]. An online solid-phase extraction (SPE) (MF Ph-1)-LC-MS/MS method, was developed to quantify olanzapine, fluoxetine, and norfluoxetine in human plasma [18].

5.2.2.2

Protein-coated silica

Protein-coated silica RAM phases consist of a protein-like human plasma protein, a1-acid glycoprotein (AGP), or bovine serum albumin (BSA), covalently bound to a reversed phase (RP) sorbent. This makes the external hydrophilic surface (protein network) of the particles compatible with a protein sample, which cannot penetrate the small pores, Fig. 5.1B. Hydrophobic groups at the inner surface are responsible for interacting with the analytes [2,3]. In 1994, Hermansson and Graham introduced extraction supports bearing AGP for direct injection of biological fluids. These supports are commercialized under the trade name BioTrap [2]. An automated method involving BioTrap 500C18 cartridges was developed to determine free noncholesterol sterols in human serum (treated serum) by online SPE-LC-MS [19]. In this method, a lipid extraction step (BligheDyer method) is essential to determine sterols even when a RAM cartridge is used in the online system. The sample preparation time is reduced by 75%, and 25 samples can be handled in only 2 h. Matrix effects have been thoroughly studied and sufficiently suppressed that external standard calibration can be used [19]. BSA has been used to provide the external hydrophilic layer, resulting in a RAMRP-BSA extraction support. On the basis of the protocol described by Menezes and Felix [20], BSA can be immobilized in situ by reaction with glutaraldehyde as the crosslinking agent, which is followed by a washing step with sodium borohydride solution. The BSA amine groups are interconnected by glutaraldehyde, which affords a crosslinked BSA layer that is unbound to the silica core. Two multidimensional LC methods (fluorescence detector) based on chiral and achiral separations have been described for direct analysis of microsomal fractions obtained from rat livers. Phenyl (RAM-phenyl-BSA) and octyl (RAM-C8-BSA) restricted access media columns were evaluated in the first dimension for sample clean up [21]. Table 5.1 describes different methods using RAM-RP-BSA columns (first dimension) for column-switching liquid chromatography. These methods share the advantages of lower organic solvent consumption (environmental friendly methods) and short analysis time. RAM-RP-BSA columns present high protein exclusion capability and appropriate sorption retention for acidic, neutral, and basic compounds [22,23]. A BSA shield eliminates macromolecules more efficiently because the negative charge density on the polymer surface increases when the medium pH is higher than the BSA isoelectric point (4.7) [4].

5.2.2.3

BSA-coated carbon nanotubes

Carbon nanotubes (CNTs) that are useful extraction sorbents due to their high surface area and mechanical strength allow p-p interactions to be established, and easy chemical modification [24]. However, protein can be retained on CNT surfaces, reducing

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their adsorption capacity and prevent their use in direct analysis of biological fluids. In 2015, Figueiredo et al. synthesized the first CNT-based RAM sorbent, which was called restricted access carbon nanotubes (RA-CNTs) [25]. This material was obtained by modifying the CNTs with a chemically crosslinked BSA layer. When RA-CNTs are used, proteins in the biological sample are excluded due to electrostatic repulsion between sample proteins and the BSA coating. RA-CNT columns (first dimension) were evaluated for direct column-switching LC analysis of antihypertensive drugs and some of their metabolites in human serum [26], anticonvulsants in human plasma [27], and tetracyclines in bovine milk [28]. The RA-CNT sorbent can exclude about 100% of the proteins (bovine milk and plasma samples) in less than 2 min [26,28] and has a long lifetime (about 250e300 analytical cycles) [26,27].

5.2.2.4

BSA-coated polymer

A novel RAM sorbent for direct analysis of enrofloxacin and gatifloxacin in milk samples was prepared by combining the hydrophilic polymer poly(glycerol monomethacrylate) (pGMMA) and BSA as the crosslinking agent. During preparation, poly(styrene-co-divinylbenzene) and pGMMA were successively grafted on silica by atom transfer radical polymerization. Then, BSA was adsorbed onto the material and crosslinked through an in-column process. The protein exclusion efficiency was higher than 99.3%. The RAM column good stability and was used for at least 5 months without significant changes in its efficiency [29].

5.2.3

Restricted access material combined with molecularly imprinted polymers (RAMIP)

Molecularly imprinted polymers (MIPs) are biomimetic synthetic materials (microporous matrix) with microcavities (binding sites) having a three-dimensional structure that can recognize target compounds (used as a template) in terms of size, shape, and chemical functionality [4]. However, when these sorbents are applied to biological samples, the MIP hydrophobic surface can adsorb nonspecific proteins from biological samples. Different strategies have been evaluated to add a protective hydrophilic coating onto MIP surfaces. In this context, the RAMIP sorbent combines the high selectivity of MIPs and the ability of RAMs to exclude macromolecules, thereby avoiding macromolecule adsorption, Fig. 5.1C. Basically, to obtain a RAMIP sorbent, the MIP surface can be covered with (1) hydrophilic monomers, (2) comonomers, followed by additional chemical treatment to convert these comonomers into their hydrophilic form, or (3) hydrophilic monomers associated with a crosslinked BSA layer [4]. Table 5.1 summarizes the main studies reporting RAMIPs for online sample preparation.

5.2.3.1

RAMIP synthesis using hydrophilic monomers

In Haginaka’s pioneering work [30], a uniform-sized MIP with a hydrophilic external layer was prepared by the multistep swelling and polymerization method.

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(S)-naproxen (template), 4-vinylpyridine (functional monomer), and EDMA (crosslinker) were added after the third swelling step to create the MIP material. The RAM layer was grafted on the MIP surface by polymerization of the hydrophilic monomers GMMA and glycerol dimethacrylate (GDMA), which were added to the reaction medium. The resulting RAMIP particles can selectively extract (S)-naproxen and exclude macromolecules from serum samples [30]. The general concepts of RAMIP sorbent multistep swelling and polymerization were applied for the determination of 2-arylpropionic acid derivatives [31], b-blockers [32], endocrine disruptors [33,34], nonsteroidal antiinflammatory drugs [35], antiepileptic drugs [36], and methylthiotriazine herbicides [37]. Biological fluids [31,33] and environmental [34e37] samples were directly injected into the chromatographic system, using RAMIP sorbents. More recently, the reversible addition-fragmentation chain transfer radical polymerization technique (RAFT) was reported to incorporate hydrophilic monomers into the MIP surface [38,39]. This methodology uses a chain transfer agent (RAFT agent; e.g., thiocarbonylthiol compounds) to ensure a controlled and milder radical polymerization [40e42]. RAFT polymerization was employed to prepare RAMIP to determine clenbuterol in biological samples [39]. This RAMIP column presented higher mechanical strength and could be used for 15 months without losing extraction/exclusion efficiency. Yang et al. [38] evaluated two types of RAFT agent, dibenzyltrithiocarbonate and cumyl dithiobenzoate, to prepare a bifunctional pyrazosulfuron-ethyl imprinted RAMIP sorbent to determine sulfonylurea residue in soil samples. The authors concluded that cumyl dithiobenzoate yields sorbents with the best extraction/exclusion efficiency.

5.2.3.2

RAMIP synthesis using comonomers

Unfortunately, in some cases, the addition of hydrophilic monomers to the reaction medium can decrease the molecular imprinting process efficiency. Hydrophilic monomers possess active hydroxyl groups that can establish hydrogen bonds with some types of template molecules or functional monomers. In this case, the prepolymerization complex can be affected during the polymerization process. To overcome this drawback, prohydrophilic comonomers followed by their conversion to their hydrophilic form is more convenient. GMA is the most commonly used prohydrophilic comonomer [43]. To illustrate this approach, Pouci et al. [44] developed a RAMIP sorbent to determine p-acetaminophenol in gastrointestinal simulating fluids, and Xu et al. [45] synthesized a MIP polymer on the surface of modified silica gel particles to determine sulfonamides in bovine milk. In both cases, after the poly(GMA) was grafted on the surface of MIP-silica particles, it was converted to GMMA via hydrolysis. During the grafting process, the pGMA chain length was controlled by the reaction time. A higher grafting density was obtained by this approach. Yang et al. [46] used an ethylenediamine/tetrahydrofuran mixture to open the epoxide groups grafted on pGMA instead of the conventional hydrolysis procedure. This reaction

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generates a monolithic RAMIP sorbent with weak ion exchange properties. This sorbent was used to determine oxacillin and cloxacillin in human urine and plasma.

5.2.3.3

RAMIP synthesis with hydrophilic monomer combined with a BSA layer

Besides hydrophilic monomers, other protective coatings, such as proteins anchored around the polymer, can also lead to good macromolecule exclusion. RAMIP sorbents using a dual RAM layer consisting of hydrophilic monomers combined with a crosslinked BSA layer have been reported [47e50]. This methodology was used to develop a RAMIP sorbent to determine chlorpromazine in human plasma [47]. After synthesizing the MIP particles, GDMA and HEMA monomers were used to create a hydrophilic surface on the MIP particles. Then, the hydrophilic surface was covered with a BSA layer, crosslinked by means of glutaraldehyde monomers. This study revealed that the macromolecule exclusion efficiency increased when the hydrophilic monomers were combined with the crosslinked BSA layer. Additional studies used the same methodology to ascertain the RAMIP (hydrophilic monomers/BSA) efficiency for antidepressants [48] and ivermectin [49] in biological samples. Santos et al. [50] published a comparative study evaluating the extraction/exclusion efficiency of different RAMIP sorbents. It was concluded that both hydrophilic monomers and BSA layer contribute to improve molecular recognition in an aqueous medium. Moreover, the authors attested that hydrophilic monomers combined with a BSA layer on the MIP surface lead to the best macromolecule exclusion. When the sample pH is higher than the protein isoelectric point, the BSA layer and the sample proteins are negatively charged, which results in electrostatic repulsion and avoids protein adsorption. At the same time, low-molecular-weight analytes diffuse through the BSA chains and bind to selective sites in the polymer. However, hydrophobic lowmolecular-weight interferents that diffuse through the BSA layer cannot adsorb onto the hydrophilic MIP surface [48,49].

5.2.3.4

Nonconventional RAMIP material

Besides the main synthetic strategies mentioned above, alternative approaches have been employed to design innovative RAMIP sorbents. Hua et al. [49] reported a novel RAMIP obtained by incorporating sugar groups into MIP particles to create a hydrophilic surface. The authors used a modified multistep swelling and polymerization method to prepare this sorbent. The modification consisted of using lactose octa-acetate functionalized with polymerizable groups. Through copolymerization of this modified molecule, after the third swelling step, the glycopolymer was incorporated into the MIP polymer. The results revealed that the recognition sites for phenobarbital remained unchanged after modification with the glycol layer. The biocompatible RAMIP sorbent presented more than 90% efficiency for protein exclusion.

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Figure 5.2 (A) single column mode and (B) coupled-column mode system configuration.

5.3

RAM sorbents interfaced with LC systems

After the synthesis procedure, RAM sorbents need to be packed in an appropriate column before they are coupled to the chromatography system. Normally, particulate sorbents are packed in stainless-steel columns (w10  4.0 mm I.D.; 50  4.6 mm I.D.) by means of the slurry technique, during which the particles are homogeneously suspended in an appropriate liquid to avoid particle-to-particle bridging or sedimentation. Then, this suspension is forced into the stainless-steel column under pressure and at a high flow-rate [51,52]. These RAM columns allow complex matrixes to be directly injected into the chromatography system. Typical instrument set-ups are (1) the single column mode, in which the RAM column is directly connected to a detector, and (2) the column-switching mode, in which a switching valve connects the RAM column to the separation column (Fig. 5.2).

5.3.1

RAM sorbents in the single column mode

In the single column configuration, the RAM column is connected directly to a detector. The sample is injected into the column for the extraction of target compounds and the exclusion of macromolecules. Subsequently, the analytes are eluted from the column to the detector. The single column mode has more promising applications when it is combined with mass spectrometric detectorsdthe high sensitivity and selectivity of these detectors has significant advantageous over UV, DAD, or fluorescence detectors. Moreover, the mass resolving power of the mass spectrometer compensates to some

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extent for incomplete analyte separation. Therefore, shorter RAM columns and higher flow rates can be used, reducing analysis times [53,54]. RAM columns direct coupling to MS/MS systems, for example RAMIP-MS [55] and a RAM-MS/MS [23] methods were developed for tricyclic antidepressants and fluoroquinolones, respectively. The RAMIP-MS and RAM-MS/MS methods provided a lower limit of quantification (LLOQ) of 15,000 and 20 ng mL1, respectively.

5.3.2

RAM sorbents hyphenated with column-switching and multidimensional systems

Column-switching and multidimensional systems have been used with two or more columns. The target fractions are selectively transferred online from the first column to a second column for further separation [56]. Two approaches have been described: the column-switching and the multidimensional methods. In the column-switching approach, the RAM column is installed in the six-port valve or sometimes placed in the loop position. Target analytes in aqueous samples are extracted (partition or adsorption) and concentrated on the RAM column. Subsequently, the extracted analytes are directly desorbed from the RAM column by the initial flow of the mobile phase (dynamic desorption) after the six-port valve (inject mode) is switched. Finally, the desorbed analytes are transported to the LC column for separation and detection [57]. In this context, many LC (in the columnswitching mode) methods using packed RAM columns have been reported for different analytes by direct injection of environmental and biological samples (Table 5.1). Typically, the use of RAM columns containing fully porous silica particles with alkyl [5e12] or phenyl [21] groups, porous silica with alkyl groups combined with a glycoprotein [29] or BSA [19,22] layer, reversed-phase sorbents using ADS particles [5e12], CNTs combined with crosslinked BSA [26,27], and RAMIPs [31,32,36e39,45e50,58] have been reported. These methods were validated for LC-UV [16,21,27,31,32,37e39,45e49,58], LC-DAD [15,22,28,29], LC-FD [11,17], LC-MS [36,50], LC-MS/MS [5e7,9,18,19,23,26,59], LC-MS/MS/MS [8], and GC-MS [10] systems. In the multidimensional approach, two or more independent separation modes are combined to separate complex mixtures. The main focus of this approach is to obtain chromatographic separation on both columnsdon the RAM column, in the first dimension, and on the separation column, in the second dimension. Regarding the separation method, a single desired segment (heart-cutting mode) or all the eluent (comprehensive mode) from the first column is transferred to the second column for further separation, which typically involves a different mechanism. A switching valve is used to couple the two dimensions [57]. Multidimensional methods with different column combination such as RAM (Phenyl-BSA)  chiral column [21], RAM (SO3-Diol)  reversed phase (C18) [14], RAM (Alkyl-Diol)  reversed phase (C18) [16], RAM (polyvinyl alcohol)  HILIC [60], RAM (C18-ADS)  normal phase (pentafluorophenylpropyl linkage) [11], and RAM (hydrophobic-hydrophilic monomers)  reversed-phase (C8) [12] have been reported to determine target analytes in complex samples.

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5.4

Solid-Phase Extraction

Conclusion and future challenges

Online column-switching or multidimensional (heart cutting or bidimensional) LC methods with RAM sorbents not only enhance analytical accuracy and precision but also decrease the matrix effect. This chapter aimed to emphasize innovative RAM sorbents; that is, hybrid materials with BSA coating, monolithic phases, carbon nanotubes, and molecularly imprinted polymer. Tuning of the sorbent structure leads to significantly enhanced selectivity and sensitivity for online LC methods. Future challenges are related to interfacing RAM columns with miniaturized chromatographic systems (CapLC and NanoLC) or their direct coupling to MS/MS systems.

Acknowledgements The authors would like to acknowledge FAPESP (Fundaç~ao de Amparo a Pesquisa do Estado de S~ao Paulo, process 2017/02147e0 and 2016/01082e9) and INCT-TM (465458/2014e9) (Instituto Nacional de Ciência e Tecnologia Translacional em Medicina).

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[45] Xu W, Su S, Jiang P, Wang H, Dong X, Zhang M. Determination of sulfonamides in bovine milk with column-switching high performance liquid chromatography using surface imprinted silica with hydrophilic external layer as restricted access and selective extraction material. J Chromatogr A 2010;1217:7198e207. https://doi.org/10.1016/ j.chroma.2010.09.035. [46] Yang G, Feng S, Liu H, Yin J, Zhang L, Cai L. On-line clean-up and screening of oxacillin and cloxacillin in human urine and plasma with a weak ion exchange monolithic column. J Chromatogr B 2007;854:85e90. https://doi.org/10.1016/j.jchromb.2007.04.001.  Florenzano FH, [47] De Oliveira Isac Moraes G, Da Silva LMR, Dos Santos-Neto AJ, Figueiredo EC. A new restricted access molecularly imprinted polymer capped with albumin for direct extraction of drugs from biological matrices: the case of chlorpromazine in human plasma. Anal Bioanal Chem 2013;405:7687e96. https://doi.org/10.1007/s00216013-7275-5. [48] da Silva KKMS, Boralli VB, Wisniewski C, Figueiredo EC. On-line restricted access molecularly imprinted solid-phase extraction of selective serotonin reuptake inhibitors directly from untreated human plasma samples followed by HPLC-UV analysis. J Anal Toxicol 2016;40:108e16. https://doi.org/10.1093/jat/bkv121. [49] de Lima MM, Vieira AC, Martins I, Boralli VB, Borges KB, Figueiredo EC. On-line restricted access molecularly imprinted solid phase extraction of ivermectin in meat samples followed by HPLC-UV analysis. Food Chem 2016;197:7e13. https://doi.org/ 10.1016/j.foodchem.2015.10.082.  Figueiredo EC. [50] Santos MG, Moraes G de OI, Nakamura MG, dos Santos-Neto AJ, Restricted access molecularly imprinted polymers obtained by bovine serum albumin and/ or hydrophilic monomers’ external layers: a comparison related to physical and chemical properties. Analyst 2015;140:7768e75. https://doi.org/10.1039/C5AN01482D. [51] Chapter 5 silica columnsepacking procedures and performance characteristics. 1979. p. 169e86. https://doi.org/10.1016/S0301-4770(08)60809-X. [52] Claessens HA, Aben G, Vonk N. Packing procedure of silica columns for HPLC with aqueous slurries. J High Resolut Chromatogr 1982;5:250e4. https://doi.org/10.1002/ jhrc.1240050505. [53] Mullett WM. Determination of drugs in biological fluids by direct injection of samples for liquid-chromatographic analysis. J Biochem Biophys Methods 2007;70:263e73. https:// doi.org/10.1016/j.jbbm.2006.10.001.  [54] Huclova J, Satínský D, Maia T, Karlícek R, Solich P, Ara ujo AN. Sequential injection extraction based on restricted access material for determination of furosemide in serum. J Chromatogr A 2005;1087:245e51. https://doi.org/10.1016/j.chroma.2004.11.055. [55] Santos MG, Tavares IMC, Barbosa AF, Bettini J, Figueiredo EC. Analysis of tricyclic antidepressants in human plasma using online-restricted access molecularly imprinted solid phase extraction followed by direct mass spectrometry identification/quantification. Talanta 2017;163:8e16. https://doi.org/10.1016/j.talanta.2016.10.047. [56] Kataoka H, Saito K. Recent advances in column switching sample preparation in bioanalysis. Bioanalysis 2012;4:809e32. https://doi.org/10.4155/bio.12.28. [57] Costa Queiroz ME, Donizeti de Souza I, Marchioni C. Current advances and applications of in-tube solid-phase microextraction. TrAC Trends Anal Chem 2019;111:261e78. https://doi.org/10.1016/j.trac.2018.12.018. [58] Hua K, Zhang L, Zhang Z, Guo Y, Guo T. Surface hydrophilic modification with a sugar moiety for a uniform-sized polymer molecularly imprinted for phenobarbital in serum. Acta Biomater 2011;7:3086e93. https://doi.org/10.1016/j.actbio.2011.05.006.

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149

[59] Li D, Zhang Z, Yang X, Zhou C, Qi J. Online restricted-access material combined with high-performance liquid chromatography and tandem mass spectrometry for the simultaneous determination of vanillin and its vanillic acid metabolite in human plasma. J Sep Sci 2016;39:3318e26. https://doi.org/10.1002/jssc.201600466. [60] García-Gomez D, Rodríguez-Gonzalo E, Carabias-Martínez R. Design and development of a two-dimensional system based on hydrophilic and reversed-phase liquid chromatography with on-line sample treatment for the simultaneous separation of excreted xenobiotics and endogenous metabolites in urine. Biomed Chromatogr 2015;29:1190e6. https://doi.org/ 10.1002/bmc.3407.

Aptamer-based and immunosorbents

6

Valérie Pichon 1,2 1 Department of Analytical, Bioanalytical Sciences and Miniaturization, UMR CBI, ESPCI Paris, PSL Research University, Paris, France; 2Sorbonne University, Paris, France

6.1

Introduction

Regardless of technological advances leading to a gain in sensitivity of analytical instruments, a sample processing step is generally required to extract and isolate compounds from environmental samples, biological fluids, or food matrices. Solid-phase extraction (SPE) is nowadays one of the most widely used methods for sample preparation. Conventional SPE sorbents are based on hydrophobic, hydrophilic, or ionic interactions that allow the retention and extraction of the target compounds from the liquid samples or extracts before their analysis by the separation methods. However, while high extraction recoveries can be expected owing to the large variety of sorbents available in various formats, the coextraction of the compounds also present in the sample can lead to coelution with the analytes of interest during the separation step, which prevents their quantification. In order to overcome the lack of selectivity of these sorbents, other sorbents involving antibodies or aptamers, thus promoting a selective retention mechanism based on molecular recognition, have been developed. Antibodies are proteins produced in the mammalian immune system in response to the presence of a foreign substance (called antigen). Among the five major types of immunoglobulins (IgA, IgD, IgD, IgE, IgG, and IgM), IgGs are the simplest ones with a molecular weight of about 150 kDa. They contain two identical heavy chains and light chains (containing 450e650 and 214 amino acids respectively) that are covalently linked by Y-shaped disulfide bonds in and forming two identical antigenbinding sites located at the upper ends of the Y structure (Fig. 6.1A). A large number of interactions (hydrogen bonds, ionic, hydrophobic, and Van der Waals forces) and an appropriate geometric adjustment between the binding site of the antibody and the antigenic determinant provide an antigen-antibody complex that is formed with dissociation equilibrium constant from nM to pM. Aptamers are single-stranded oligonucleotides usually between 20 and 100 bases long and may have, like antibodies, a specificity toward a ligand (small organic molecules, peptides, nucleic acids, proteins, intact cells). They have a specific and complex 3-D shape, characterized by various elements, such as stems, loops, bulges, hairpins, pseudoknots, triplexes, or quadruplexes [1], which allows them to be bound to a wide variety of targets (Fig. 6.1B). Indeed, they can be generated against various targets, such as divalent metal ions, small organic molecules, proteins, and cells.

Solid-Phase Extraction. https://doi.org/10.1016/B978-0-12-816906-3.00006-6 Copyright © 2020 Elsevier Inc. All rights reserved.

152

Solid-Phase Extraction

Figure 6.1 Representation of the specific entrapment of a compound by (A) an immunosorbent (IS) or (B) an oligosorbents (OS), (C) selective SPE procedure on IS/OS and (D) expected extraction profiles by comparing the retention of the targeted analyte specifically retained on IS or OS this retention of compounds from the matrix sample.

They are selected from a random bank containing up to 1015 different sequences by an in vitro combinatorial selection method called Systematic Evolution of Ligands by EXponential enrichment (SELEX) according to their ability to recognize a target [2,3]. Once the sequence is selected, aptamers are produced by a chemical process without requiring mammals. Some aptamers are characterized by dissociation equilibrium constants similar to those of antibodies as recently reported for different types of target molecules [4] and in particular for marine biotoxins [5]. While antibodies have been widely used for the development of bioassays, such as the well-known ELISA methods or sensors, the immunosorbents have already largely proven their effectiveness in selectively retaining or extracting different types of molecules in terms of size and of physicochemical properties. Indeed, many studies have described their potential in separation (chromatography, electrophoresis) [6,7] and solid-phase extraction [8e11]. Numerous immunosorbents are now available on the market, and their use has been validated for numerous applications, particularly for food analysis [12]. Various analytical aptamer-based methods have been developed, including enzyme-linked oligonucleotide assay (ELONA or ALISA, a variant of ELISA with aptamers instead of antibodies) [3] or biosensors (“aptasensors”) [5,13,14]. The use of aptamers as SPE sorbents (i.e., of oligosorbents) is more recent than the use of antibodies but seems to be a very promising approach [4,15,16]. This chapter describes the development, characterization, and application of both sorbents to the selective extraction of target analytes from real samples. Thus, the

Aptamer-based and immunosorbents

153

principle, advantages, limitations, and complementaries of these sorbents will be presented. The introduction of these selective tools in miniaturized devices will also be discussed.

6.2

Grafting of antibodies and aptamers on a solid support

When used as selective tools for the extraction of target molecules, antibodies and aptamers must be bound to solid supports (particles of different sizes efrom large beads to nanoparticles, rods, fibers, internal capillary walls, chip channel surfaces, etc.) according to the method to be developed. These supports must possess a chemical and biochemical inertness, good mechanical stability, and homogeneity in terms of particle size or surface. They must be easily activated to allow effective binding of biomolecules and have large pore sizes, particularly to promote the accessibility of antibodies, which are large molecules compared to aptamers (10e15 nm vs. 1e2 nm). They must be hydrophilic in order to avoid any nonspecific interactions. Finally, the immobilization procedure must preserve the affinity of antibodies/aptamers toward their target analyte. If the trapping of the antibodies by their immobilization in the pores of a hydrophilic glass matrix has been proposed, the covalent grafting of the antibodies onto a sorbent by coupling accessible chemical functions of antibodies amino acids with reactive groups on the surface of the support is the most frequently described approach. This type of covalent immobilization on traditional supports, including silica, agarose, cellulose, and synthetic polymers, such as polymethacrylate derivatives, is often based on the reaction between the amino groups of lysine residues of antibodies. This leads to random immobilization of antibodies that can affect the accessibility of the antigen to their binding sites [9]. Concerning aptamers, their chemical synthesis makes it possible to introduce modifications at the 50 or 30 end of the oligonucleotide sequence to facilitate their immobilization. This modification is chosen according to the nature of the bonding. Modification by an amino group has often been reported resulting in immobilization procedures similar to those applied for the random immobilization of antibodies with the advantage that the aptamer sequence is oriented directly toward the analyte. In addition, a spacer arm can be introduced to maintain the binding properties of the aptamer when it is bounded to a surface. It may be an n-alkyl chain in C6 or C12, an ethylene glycol derivative, as illustrated by the data reported in Table 6.1. The aptamers have also been functionalized by thiol-groups, thus favoring their binding to a gold surface or, as often described for antibodies, to biotin for achieving a noncovalent grafting to streptavidin activated particles that are commercially available. Nevertheless, noncovalent binding procedure presents some drawbacks, in terms of reusability and sorbent life-time, particularly when using organic modifiers in the SPE procedures that affect the biotin-streptavidin affinity [24,41]. Several studies have attempted to optimize the antibody immobilization through oriented binding procedures by exploiting the carbohydrate moieties located on the

Target analyte

Sample

154

Table 6.1 Applications of aptamers in extraction devices for the analysis of targets in real samples. Amount or dimension of the extraction device/Vsample

Sorbent and reagent used for the grafting

Aptamer modification/ Spacer

Ref.

Off-line SPE Aflatoxin B2

Peanut extract (diluted)

60 mg/5 mL

CNBr-sepharose

NH2/C7

[17]

CEA

Serum

4 mL (PDMS chip)/200 mL

Carboxylated-polystyrene beads (40e50 mm), NHS

NH2/-

[18]

Cocaine

Plasma (diluted 2)

35 mg/200 mL

CNBr-sepharose

NH2/C6

[19]

Post mortem blood (after PP)

35 mg/100 mL

NH2/C12

[20]

Cocaine þ diclofenac

Drinking water

50  8 mm i.d./0.2e1 L

CNBr-sepharose

NH2/C6

[21]

Ochratoxin A

Wheat extract

300 mL in cartridge/ 10e12 mL

DADPA (noncovalent)

/

[22,23]

Red wine

35 mg/1 mL

CNBr-sepharose

NH2/C6

[24]

NH2/C12

[25]

Wheat extract (diluted 10) 200 mL/3 mL

NHS-sepharose

NH2/C6

[26]

Plasma (after PP) and urine (diluted 3)

35 mg/200 mL

CNBr-sepharose

NH2/C6

[27]

Solid-Phase Extraction

Tetracycline

Ginger powder (diluted 10)

0.53 mm i. d./100 mL

Poly(TMOS-co-g-MAPS) monolith

SH/C6

[28]

Human serum (diluted 50)

0.53 mm i. d. 2 cm/1 mL

Poly(AEAPTES-co-TEOS) monolith modified with AuNP

SH/C6

[29]

OH-deoxyguanosine

Urine (diluted 5)

200  0.5 mm i. d./2 mL

Amino-mNP, GTD

NH2/-

[30]

Ochratoxin A

Beer (diluted 2)

100  0.1 mm i.d./100 mL

POSS-acrylate-based monolith (one-pot synthesis)

SH/-

[31]

Beer, wine (diluted 9)

10 cm  75 mm i. d./20 mL

Poly(TMOS-co-MPTMS) monolith modified with AuNPs (25 nm)

SH/C6

[32]

Beer (diluted 2)

70  100 mm i. d./250 nL

Poly(APTES-co-TEOS) monolith, GTD

NH2/C12

[33]

Beer and white wine (diluted 2)

15 mm  75 mm i. d./ 2.65 mL

Poly(TMOS-co-MTMS) monolith, VTMS

SH/-

[34]

On-line SPE/LC

Aptamer-based and immunosorbents

Human serum

Thrombin

On-line SPE/nanoLC Ochratoxin A

In-line SPE/CE Ochratoxin A

On-column concentratione direct injection of real samples Adenosine

Dialysated from rat cortex

5 cm  150 mm i. d./4 mL

Streptavidin activated - porous glass beads

Biotin/TEG

[35]

Cytochrome C

Serum (diluted 10)

8.5 cm  100 mm i. d./-

Streptavidin- modified poly(GMA-co-TRIM) monolith

Biotin/-

[36]

155

Continued

156

Table 6.1 Applications of aptamers in extraction devices for the analysis of targets in real samples.dcont’d Sorbent and reagent used for the grafting

Aptamer modification/ Spacer

Ref.

Target analyte

Sample

Amount or dimension of the extraction device/Vsample

Doxorubicin, epirubicin

Serum, urine

10 cm  75 mm i. d./20 mL

Poly(TMOS-co-g-MAPS) monolith (one-pot synthesis)

SH/-

[37]

Lysozyme

Chicken egg white

10  4.6 mm i. d./5 mL

Poly(GMA-co- EDMA, ethylenediamine, GTD

NH2/C6

[38]

L-selectin

CHO cellconditioned medium

5  0.5 mm i. d./-

Streptavidin-polyacrylamide resin

Biotin/-

[39]

Thrombin

Serum (diluted 20)

250 mm i. d./-

Poly(APTES-co-TEOS) monolith, GTD

NH2/C6

[40]

Solid-Phase Extraction

Aptamer-based and immunosorbents

157

heavy chains, at the lower ends of the Y structure (i.e., Fc region) after the mild oxidation of these moieties in aldehyde residues, which may react with a sorbent containing a hydrazide or amine. The use of antibody fragments was also recently proposed to limit their size while facilitating their oriented grafting. This approach seems particularly interesting for the development of miniaturized extraction devices. It must provide greater antigen-binding capacity because immobilization occurs far from the antigen-binding sites and results in a more oriented reaction. However, despite the theoretical advantages of the oriented immobilization procedure, the greater binding capacity generally observed using random immobilization that can result from a higher number of active groups greatly reduces its interest. This oriented immobilization can also be ensured by the use of a sorbent activated by proteins A or G, which also bind the antibody in the Fc region. This noncovalent immobilization strategy is based on the high affinity between the IgGs and these proteins but can be easily disrupted by lowering the pH or using an organic solvent, thus affecting the reusability of the resulting ISs as for streptavidin activated supports. A recent review on miniaturized immunoextraction-based sorbents summarized, in detail, the different approaches proposed to improve antibodies immobilization based on this reduced-size extraction format [42].

6.3

Extraction procedure using IS and OS

During the extraction step on the immunosorbent (IS), the antibody-antigen complex is formed primarily by electrostatic forces, which attract and orient both entities. Then, this causes the formation of secondary hydrogen bonds, bringing the molecules closer together, and excluding water. Finally, Van der Waals forces are initiated to form a stable noncovalent bond. The establishment of these interactions is promoted in the aqueous media and then well-adapted to aqueous samples (real waters, biological fluids). For other types of samples, such as organic extracts of solid samples, dilution of the extracts with an aqueous buffer is recommended [8]. Analytes are further recovered by the disruption of these complexes that occurs by applying conditions that may differ from the nature of the target. Indeed, the desorption of proteins is often carried out by chaotropic agents (chloride, iodide, perchlorate, and thiocyanate ions at concentration between 1.5 and 8 mol/L), temperature increase, pH variations, and the use of denaturing agents, such as water-organic solvent mixtures being applied to the desorption of small size targets [8,9]. For oligosorbents (OSs), effective trapping of the target analyte is promoted by a sample whose composition is close to the composition of the buffer used during the selection of aptamers (often called the binding buffer (BB)) to favor the interactions between both entities. The strength of the binding between an aptamer and its target, which is highly dependent on the conformation of the aptamer molecule, may be affected by a variety of cations. As an example, it is well known that some guanosine-rich aptamers can adopt inter or intramolecular quadruplex structure that is stabilized by the presence of G-quartets (square arrangement of guanines) that are

158

Solid-Phase Extraction

favored by the presence of Kþ as often reported for thrombin aptamers. This example shows the need to study the effect of different cations to determine the optimal conditions for strong binding of the target on the OS during the extraction step. Some parameters, such as temperature [43e47], ionic strength, and pH [43] also strongly affect the conformation of aptamers. These parameters must be controlled and adapted to ensure high extraction recoveries or favor the elution [48]. Different approaches are possible to induce elution: water-organic modifier mixtures and chaotropic agents such as NaClO4, denaturing agents such as urea or guanidine hydrochlorides for antibodies, and scavenging agents, such as EDTA for aptamers sensitive to cations, temperature increase, and pH variations. The choice of those conditions strongly depends on the nature of the analyte(s) and of the aptamers. The use of organic solvents never reported for antibodies (such as hexane [49], a hexane-ethyl acetate mixture [50] or dichloromethane [51,52]) was reported without preventing the reusability of the OS. The addition of trypsin for the direct digestion of a target protein [53] or of recombinant DNase that damages the aptamers [54] have also been described. When not in use, both ISs and OSs are stored at 4 C in a buffer solution, a PBS buffer for ISs and the BB for OSs. Sodium azide, a bacteriostatic agent, is generally added in the storage buffer. This solution can be removed by pure water or by renewing BB solution during the conditioning step.

6.4

Various extraction format and methods

Immunoaffinity cartridge of 1e3 mL is the most commonly used format for immunoextraction [8,9,12]. Many oligosorbents were also developed in this format too. However, during these last years, IS and OS available in other formats and allowing the development of other extraction methods than SPE have been developed and are described below.

6.4.1

SPE in cartridge or column

Many ISs have been developed since the early 1990s in cartridges to carry out off-line SPE [8], such as the commercialized ISs dedicated to mycotoxins analysis, which constitute the most important ISs market [12]. For application to real samples, 30e60 mg of ISs are generally packed between two frits into disposable cartridges or columns as a conventional sorbent for SPE as shown in Fig. 6.1C. The advantage of the cartridge format is that it is adapted to all the consumable and automated methods developed for conventional SPE sorbents. For OSs, their application in SPE to real samples is more recent. Those works are summarized in Table 6.1. As commercially available ISs, most of the OSs were prepared by immobilizing aptamers on activated Sepharose beads and by applying a similar extraction procedure. After conditioning them with a few ml of aqueous media that favors the interaction of the target analyte with the antibodies/aptamers, the sample is percolated. After a washing step that can be applied to remove some matrix components slightly retained

Aptamer-based and immunosorbents

159

during the sample percolation, the target analyte is then desorbed by a solution affecting the interactions with the antibodies/aptamers. As schematized in Fig. 6.1D, matrix components, not recognized by antibodies/aptamers, and therefore, not retained on the IS/OS must be removed after the washing step, thus giving rise to a final elution fraction that contains only the targeted analyte. An important parameter in this sequence, as in conventional SPE procedure, is the volume that can be percolated without loss of the analyte during this percolation step to ensure high recoveries. This volume is highly dependent on the affinity of the antibodies/aptamers for the target analyte(s). It can be drastically reduced by choosing a percolation medium that does not promote this affinity, as previously mentioned. The dilution of the real samples with a buffer is often performed to limit the risk of loss of affinity caused by matrix components. If dissociation constants for antibodies and their antigens are in the nanomolar range, the Kd values for aptamers may fluctuate [2], and high Kd values limit the possibility to percolate high sample volumes and then to reach high enrichment factors. This was well illustrated by Hu et al. who reported the possibility to percolate up to 2 L of drinking water containing diclofenac without observing a decrease of recovery, while the sample volume was limited to 0.5 L of water for the extraction of cocaine. Indeed, the breakthrough volume was lower for cocaine than for diclofenac because of the higher Kd value of cocaine aptamer (Kd ¼ 5 mM) compared to diclofenac aptamer (Kd ¼ 47.2 nM) [21]. In return, for antibodies, the Kd value can be low for its antigen but higher for a structural analog, thus giving rise to a decrease of recovery for this molecule for too high percolated volume. An example of evaluation of this breakthrough volume is given in Fig. 6.2 A, which illustrates the effect of the volume of sample percolated on an anticiprofloxacin (CIP) IS (2 mL gel) on the recovery of extraction of CIP. The affinity of the antiCIP antibodies allows the percolation of up to 5.5 mL of the sample while the recovery of quinine (QUIN), not recognized by these antibodies, decreases from 2 mL of the sample [55]. The optimization of an extraction procedure also consists in applying strong elution conditions to limit the volume of eluent, as illustrated by the elution of CIP with different ratios of methanol in water (Fig. 6.2B), thus favoring high enrichment factors without requiring in some cases additional evaporation steps of the final extract. The selectivity brought by ISs and OSs is illustrated by comparing the chromatograms of elution fractions from an OS (Fig. 6.3A), an IS (Fig. 6.3C), and hydrophobic support (C18 grafted silica, Fig. 6.3B) during the purification of a cereal extract. The IS and OS allow the elimination of a large part of the interfering compounds and eliminate the visible coelution at the OTA peak. To miniaturize these selective SPE devices, the immobilization of antibodies and aptamers onto monoliths in-situ synthesized into capillaries of 75e100 mm internal diameter or chip channel has also been proposed, as illustrated in Table 6.1 for aptamers and Table 6.2 for antibodies. These miniaturized extraction devices can be directly coupled on(in)-line with CE, nanoLC or on-chip separations, thus facilitating the automation of the whole analytical procedure while reducing the volume of sample and reagent consumption. This last point is particularly important when relatively expensive reagents such as antibodies are used.

160

Solid-Phase Extraction

(A)

QUIN CIP

1.0

CC(C/C0)

0.8 0.6 0.4 0.2 0.0 0

2

1

3

4

6

5

7

8

V (mL)

(B)

60% MeOH 80% MeOH 100% MeOH

100

Recovery (%)

80 60 40 20 0 0

2

4

6 V (mL)

8

10

12

Figure 6.2 (A) Breakthrough curves of CIP and QUIN obtained in antiCIP pAb immobilized column. (A) Elution profiles of CIP with a different percent of methanol. Reproduced from Zhang XH, Deng Y, Zhao MZ, Zhou YL, Zhang XX. Highly-sensitive detection of eight typical fluoroquinolone antibiotics by capillary electrophoresis-mass spectroscopy coupled with immunoaffinity extraction. RSC Adv.2018;8(8):4063e71 with permission from the Royal Society of Chemistry.

At last, some applications of a column containing grafted aptamers and directly connected to UV detection are listed in Table 6.1. In these works, if the concentration effect cannot be expected, limiting thus the sensitivity of the method, the aptamers are used to remove the matrix components at the beginning of the chromatogram before

Figure 6.3 HPLC chromatograms resulting from the analysis of elution fractions after the extraction of a red wine sample spiked at 2 mg/L with Ochratoxin A (OTA) on the oligosorbent based on a covalent immobilization (A) on the conventional C18 silica cartridge (B) and on the immunosorbent (C). Reproduced from Chapuis-Hugon F, du Boisbaudry A, Madru B, Pichon V. New extraction sorbent based on aptamers for the determination of ochratoxin A in red wine. Anal Bioanal Chem 1 mai 2011;400(5):1199e1207. With permission from Springer Nature.

Amount or dimension of the extraction device/Vsample

Sorbent and reagent used for the grafting

162

Table 6.2 Immunoaffinity monolith coupled to CE, nanoLC or integrated on-chip and applied to real samples.

Ref

Poly (GMM-co-PETA) (diol), poly (GMA-co EDMA) (epoxy), poly GMM-co-EDMA, poly (aminopropylacrylamide hydrochloride-co- EDMA) monoliths

Reduced pAbs (aldehyde groups) grafted on diol or epoxy-based sorbent or pAbs and Fabs (amino group) grafted using GTD

[56]

4.5 cm  100 mm i. d. Capillary/ 150 nL

poly(APTES-co-TEOS) hybrid monolith

mAbs grafted using GTD

[57]

Labeled human serum

0.6 mm length/

poly(GMA-co-EGDMA) monolith polymerized in the COC chip activated with PEGdiacrylate

Covalent grafting (epoxy) of pAbs

[58]

Labeled protein (ferritin)

Human serum (Diluted 5)

0.6 mm length/

poly(GMA-co-EGDMA) monolith prepared in a 3D printed chip (45 mm x 50 mm)

Covalent grafting (epoxy) of pAbs

[59]

Labeled ProGRP digest

Human serum digest

15 cm  180 mm i. d. Capillary/ 20 mL

Poly(EDMA-co-VDM) monolith

Covalent grafting (via VDM) of mAbs

[60]

Sample

Haptoglobin

Serum

100 mm i.d. capillary/

MicrocystinLR

Algae extract

Labeled -proteins (ferritin, lactoferrin)

Solid-Phase Extraction

Antibody grafting

Target

Aptamer-based and immunosorbents

163

Figure 6.4 Chromatograms for the determination of epirubicin in the serum of breast cancer patients treated with epirubicin by intravenous injection. Facile one-pot synthesis of an aptamer-based organicesilica hybrid monolithic capillary column by “thioleene” click chemistry for detection of enantiomers of chemotherapeutic anthracyclines. Reproduced from Jiang H-P, Zhu J-X, Peng C, Gao J, Zheng F, Xiao Y-X, et al. Facile one-pot synthesis of a aptamer-based organicesilica hybrid monolithic capillary column by “thioleene” click chemistry for detection of enantiomers of chemotherapeutic anthracyclines. Analyst 2014; 139(19):4940e4946 with permission from the Royal Society of Chemistry.

the increase in the eluent strength of the mobile phase allowing the elution of the target molecule. An example is provided in Fig. 6.4. It shows the potential of this simple approach to differentiate the serum of breast cancer patients treated with epirubicin and serum from a healthy control.

6.4.2

Dispersive SPE

In the dispersive mode, dSPE, the extraction is carried out by introducing a given amount of IS/OS into the sample. After a sufficient extraction time under stirring, the particles are recovered mainly by centrifugation or magnetic field (if magnetic core particles are used) and then introduced into a suitable desorption solvent after a possible washing step. Application of dSPE methods using antibodies and aptamers are summarized in Table 6.3. In these works, antibodies/aptamers were grafted on conventional beads, magnetic beads and also nanoparticles (in the order of tens of nanometers instead of tens of micrometers). As with conventional dSPE sorbents, the key-parameters to be optimized are the sample volume/sorbent ratio, the extraction time, the stirring rate, the nature of the solvent, and the time required for desorption [77]. While some studies have reported the use of commercially available streptavidinmagnetic agarose beads that make it really easy to immobilize biotinylated antibodies/aptamers and apply them, numerous works reported the use of NPs.

Table 6.3 Some applications of ISs and OSs in dSPE for the analysis of targets in real samples.

Anatoxin A

Water

10 mL/20 mL

Epitestosterone

Urine

Human chorionic gonadotropin

164

Samples

Amount or dimension of the extraction device/Vsample

Sorbent and reagent used for the grafting

Grafting method

Ref

NHS-sepharose magnetic beads (27 mm)

Grafting of mAbs via amino groups

[61]

10 mg/20 mL

Fe3O4-Au NPs (50 nm)

Grafting through Au-S bonds of half mAb (reduction of S-S bond)

[62]

Dried blood spot, serum, urine

20 mL/1 mL

Tosylactivated magnetic beads (2.8 mm)

Grafting of mAbs (previously stored at pH 2.5)

[63,64]

Low-density lipoprotein

Plasma (diluted 1/500)

10 mL/10 mL

Au-NP (28 nm)

Noncovalent grafting; covalent grafting of mAbs to carboxy terminated-pegylated NPs (amide), oriented immobilization of mAb via oxidized carbohydrate moiety on hydrazide derivatized NP, noncovalent via cysteine-tagged protein A-NPs

[65]

Microcystins

Urine

5 mg/100 mL

Streptavidin -magnetic beads

Biotinylated antibodies

[66]

Soy proteins

Soy milk (500 mL)

23 mg/500 mL

Fe3O4-Au NPs (12 nm)

Oxydized pAbs on amino-functionalized Au-NPs

[67]

ATP

Deproteinated cell lysate

25 mL/250 mL

AuNP (13 nm)

SH- Apt

[68]

Adenosine

Urine

8 mg/200 mL

Silanized mNPs, APTES, GTD

NH2-C6- Apt

[69]

Target analyte

Immunosorbent

Solid-Phase Extraction

Oligosorbent

Milk extract (purified by PP and LLE)

16 mg/15 mL

Silanized mNPs, APTES, GTD

e

[51]

Aflatoxin B1, B2

Maize extract

1 mL/1 mL

Streptavidinmagnetic agarose beads (28 mm)

Biotin-Apt

[41]

Amphenicol antibiotics

Milk (diluted 100)

50 mg/100 mL

Silanized mNPs, APTES, EDC, Sulfo-NHS

NH2- Apt

[43]

Arsenic

Groundwater

2 mg/1 mL

Streptavidin-agarose beads

Biotin-Apt

[70]

Berberine

Herb extract

20 mg/1 mL

Avidin -activated amino-mNPs, GTD

Biotin-Apt

[50]

Bisphenol A

Human serum, urine (diluted 2) treated with enzymes

5 mg/1 mL

Avidin- mNPs

Biotin-Apt

[71]

17b-estradiol

River water

50 beads/ 100 mL

NCS-modified glass beads (250 mm), APTMS, PDITC

NH2- Apt

[48]

His6-Tag protein

E Coli lysates

5 mg/100 mL

NH2-magnetic beads, GTD

NH2-C6 or C7-Apt

[72]

Histones

Cell lysate

/

Streptavidin agarose beads

Biotin-Apt

[73]

Aptamer-based and immunosorbents

Aflatoxin M1

Continued 165

Table 6.3 Some applications of ISs and OSs in dSPE for the analysis of targets in real samples.dcont’d 166

Amount or dimension of the extraction device/Vsample

Sorbent and reagent used for the grafting

Ref

Carboxylated-mNPs (100e250 nm)þ EDC, NHS

NH2-C6- Apt

[44]

10 mg/-

Streptavidin magnetic MOF

Biotin-Apt

[74]

Soil extract

30 mg/100 mL

mNP coated by amino-mMOF, GTD

NH2-C6- Apt

[49]

0H-PCBs

Human serum

30 mg/40 mL

mNP (18 nm), APTES

NH2- Apt

[75]

Sulfanilamide

Milk

10 mL/990 mL

Magnetic carboxylated-silica particles

NH2- Apt

[45]

Thrombin

Serum, blood

/

Magnetic beads coated with AuNP

SH-C6- Apt

[53]

Plasma

100 mL/100 mL

Au-nanorods (77  17 nm)

SH-(EG)6- Apt

[76]

Bacterial lysate

2 mg/100 mL

Streptavidinmagnetic beads

Biotin-Apt

[54]

Samples

Ochratoxin A

Coffee extract (diluted 2)

100 mL/100 mL

Corn, peanut extract diluted in BB PCBs

Thyroid transcription factor 1 (TTF1)

Solid-Phase Extraction

Grafting method

Target analyte

Aptamer-based and immunosorbents

167

The works reported in Table 6.3 show that the quantities of supports used are variable, but the use of nanoparticles leads to a decrease in these quantities, and therefore, to a decrease in the quantities of aptamers and antibodies used, thus decreasing the cost of the methods while reducing sample volumes. Immuno-dSPE, also known as immunocapture, was applied to sample volumes from 10 mL to 20 mL, depending on the expected enrichment factor, with extraction time between 5 min and overnight. For OS-based dSPE, ratios of 10 mL of beads for the extraction of 100 mL of sample to 50 mg of beads dispersed in 100 mL sample with extraction times in the range 3 min to 2 h were reported. With both sorbents, recoveries higher than 80% were reported. The possibility of performing this extraction method in 96-well plates for easy automation has recently been reported [66].

6.4.3

SPME, SBSE, and associated methods

Other methods based on the equilibrium of the target between the sample and the immobilized antibodies/aptamers were developed by replacing particles by other formats, such as fibers or rods of different sizes up to thin films resulting in selective extraction methods named solid-phase microextraction (SPME), stir-bar sorptive extraction (SBSE), or thin-film microextraction (THME), respectively. A quite exhaustive view of the applications related to the use of OS and IS in these formats is provided by Table 6.4. Mostly developed for the extraction of the volatile compounds before their analysis by gas chromatography (GC), the applications of SPME have been extended to biological fluids with an off-line coupling with liquid chromatography (LC) [87]. Therefore, its combination with the high antibodies or aptamers selectivity which limits the coextraction of interfering compounds present in complex samples, such as biological fluids, can be an interesting approach to improve the sensitivity of the method. SPME generally consists in the extraction of compounds by immersing a fused silica fiber (100 mm diameter) usually coated by an organic polymer (7e100 mm thickness) into a sample, and different parameters can be optimized to extract high amounts of analytes, such as the extraction time, stirring speed, nature, and the volume of the desorption solution or the desorption time, as for conventional SPME. The same parameters have to be optimized in SBSE and THME that are only supposed to differ by the device sizes. Nevertheless, the dimensions of these different devices are not so different as demonstrated by those reported in Table 6.4, the classification in this Table being in agreement with the name of the method provided by the authors of the reported works. The preparation of the extraction devices mostly consists in the direct activation of silica-based material, cellulose paper, and graphene oxide sheet allowing the covalent grafting with NH2-functionalized aptamers or amino-groups of antibodies. The immobilization of activated particles or the polymerization of a monolith onto rods were also proposed, those approaches being used to improve the sorbent capacity, as it will be discussed later on. In most of the cases, the extraction times are about 30 and 60 min and desorption times between 20 and 40 min as for conventional SPME sorbent. Reported recoveries

Method

Target

Samples

Amount or dimension of the extraction device/ Vsample

168

Table 6.4 Applications of ISs and OSs in SPME, SBSE, and associated.

Sorbent used for the grafting

Grafting method

Ref

Immunosorbent SPME

SBSE

Theophylline

Serum (diluted with PBS, 1/100, v/v)

Fiber (1.8 mm, 2.3 cm)/-

Silanized fiber

pAbs grafted with GTD

[78]

Benzodiazepines [3]

Urine

Rods (4 mm, 2.5 cm)/-

Silanized borosilicate glass rods

pAbs purified pAbs, mAbs grafted with GTD

[79,80]

Non-steroidal estrogens [3]

Environmental waters (dilution 1/2)

Stainless steel rods (2  18 mm)/1 mL

5 mm-porous silica particles coated on rods

mAbs grafted with GTD

[81]

Quinolones [11]

Bovine milk (centrifuged to remove fat)

bars (5 mm, 3 cm)/-

Silanized borosilicate glass bars

mAbs grafted with GTD

[82]

Adenosine

Human plasma

1 cm length fiber/-

Wire (125 mm diameter) coated with poly (TMOS-co-APTES)

COOH-Apt þ EDC, NHS

[83]

Thrombin

Human plasma (diluted 20)

55  1.5 mm i.d./ 2 mL

Stainless steel rod coated with PANCMA (electrospun microfiber)

NH2-C6-Apt þ EDC, NHS

[84]

Oligosorbent SPME

Solid-Phase Extraction

PCBs

Purified (silica gel) fish extract

50  2 mm/-

Wire coated with MOF5, PDDA

Noncovalent

[52]

TFME

Codeine

Urine (diluted 100)

-/10e15 mL

Oxidized cellulose paper

NH2-C6Apt. þ NaCNBH3,

[46]

Codeine, Acetamiprid

Pharmaceutic tablets, urine

Triangle (14 mm  4 mm)/ 15 mL

NH2-C6-Apt. þ NaIO4

[47]

Methamphetamine

Saliva (diluted 10), precipitated plasma (diluted 20)

Triangle (14 mm  4 mm)/ 700 mL

Cellulose treated by carbon dots

NH2-C6-Apt. þ EDC, NHS

[85]

Thrombin

Plasma (diluted)

20 mg/1 mL

Graphene oxide sheet

NH2-C6-Apt. þEDC

[86]

Aptamer-based and immunosorbents

SBSE

169

170

Solid-Phase Extraction

are between 13% and 67% with associated RSD values lower than 14% for IS based methods and higher than 80% with RSD values lower than 10% for OS-based methods. In addition to these methods, immunoaffinity-based capillary microextraction method, also called in-tube SPME (IT-SPME) was proposed. With this approach, the analytes are extracted by antibodies fixed at the inner surface of a capillary before being desorbed and transferred directly to the separation device. It presents several advantages compared to SPME, such as an online coupling with separation methods (capillary electrophoresis (CE) and more frequently, LC). Applications of immunoaffinity IT-SPME to real samples are reported in Table 6.5. To my knowledge, no similar developments were achieved using aptamers. In 1998, Phillips and Dickens proposed an “immunoaffinity capillary electrophoresis” method that consists in the covalent grafting of antibodies fragments (Fabs) at the inner surface of approximatively 6 cm of a 100 mm silica capillary, with the remaining part of the 30 cm-capillary being used for the CE separation [88]. Since this pioneering work, other methods were proposed. They can perform based on the injection at a fixed flow-rate of a fixed volume of sample or by repeated aspirations (draw) and ejections of a fixed volume samples through the capillary. The sample draw/ejection volume and the number of draw/ejection cycles have to be adapted to the capillary capacity [89] and to the volume comprised between the injection needle and the capillary. All the extractions performed by this approach were achieved in less than 10 min, the shortest time than using the previous methods. In addition to the developments using capillaries, the coating of the surface of a chip channel was also proposed by polymerizing a thin film of a polymer on a 0.6 mm-length channel to couple IT-SPME online with a chip-based CE separation [91,92]. This device was first developed for the trapping of a single molecule, a-fetoprotein (AFP), from human serum thanks to the immobilization of antiAFP antibodies on the thin film of the polymer [91]. Four antibodies specific of four different biomarkers were further simultaneously immobilized in the same way, thus enabling the simultaneous extraction of these biomarkers from the same human serum sample, their individual identifications being achieved after their transfer to the separation channel [92].

6.5

Capacity

The capacity is defined as the maximal amount of a molecule that can be retained by the sorbent during the percolation step. For these selective sorbents, it depends on the number of active immobilized antibodies/aptamers, which is directly related to their bonding density, usually expressed in mg/mL of sorbent bed or mg/mg of the sorbent. This parameter strongly depends on the concentration of biomolecules introduced in the grafting solutions and of the specific surface area of the solid support accessible for the immobilization of the biomolecules [4,8]. Supports with small pore sizes have a high surface area, but low accessibility for the large biomolecules. On the other hand, supports with large pore sizes have good accessibility but a small surface

Dimension of the extraction device/Vsample

Extraction sorbent

Grafting method

Ref

Urine, plasma, CSF, saliva

6 cm of a 30 cm capillary, 100 mm i. d./30 nL

Activated (APTES) silica capillary

Grafting of Fabs from mAbs, SSMCC

[88]

Fluoxetine

Serum

70 cm, 250 mm i. d./ 20  50 mL

Activated (APTES) silica capillary

Covalent grafting of pAbs, GTD

[89]

Interferon a

Plasma

60 cm, 250 mm i. d./ 20  150 mL

Activated (APTES) silica capillary

Covalent grafting of mAbs, GTD

[90]

FITC-labeled AFP, four labeled-biomarkers (proteins)

Labeledhuman serum

3 mm thickness, 6 mm/10 mL

PMMA-chip channel coated with poly(GMA-coPEGDA) monolith film

Covalent grafting of Abs

[91,92]

Target analyte

Sample

Cytokines

Aptamer-based and immunosorbents

Table 6.5 Applications of immunoaffinity in-tube SPME.

171

172

Solid-Phase Extraction

area. The bonding density can be calculated experimentally by measuring the concentration of the antibodies/aptamers that remains in the binding solution by UV spectrophotometry. Nevertheless, this approach only allows the determination of the amount of grafted biomolecules, including also those that are not accessible for the target analyte. The real capacity of ISs/OSs can be determined by measuring the amount of retained target analyte as a function of its concentration in the percolated sample after applying the optimized extraction procedure. By plotting the amount of target analyte retained by the sorbent (i.e., recovered in the elution fraction) as a function of its concentration in the percolated sample, a curve is obtained that is characterized by two different parts [17,19,20,24,25]. First, for the lowest percolated amounts/concentration levels, a linear part is obtained and corresponds to a range of concentrations for which a constant recovery yield of extraction is obtained, this recovery yield being given by the slope of this linear part. Working in this range ensures the possibility to carry out quantitative analyses. In the second part of the curve, for higher percolated amount, the recovery yields decrease, and the amounts of target analyte retained by the sorbent tend to reach a plateau. This decrease in recovery yield is caused by the overloading of the sorbent capacity, i.e., the saturation of the antibodies/aptamers-binding sites. The capacity of the sorbent is given by the amount/concentration corresponding to the upper limit of the linear range. Knowing this amount, it is then possible to evaluate the part of active aptamers or antibodies among the total amount of immobilized ones, thus allowing a better evaluation of the efficiency of the grafting procedure(17,40). Concerning immunosorbents, both polyclonal (pAbs) and monoclonal (mAbs) antibodies can be used for their preparation. The pAbs are cheaper to obtain, but their production suffers from a lack of reproducibility in terms of the time of response of an animal, quantity, and even specificity. In contrast, the production of mAbs is costly but guarantees a long-term production of reproducible antibodies that does not require animals for further large-scale production. Some authors also proposed the use of purified pAbs allowing to increase the amount of specific immobilized antibodies [79,80], of Abs fragment (Fabs) [88] or of half-antibodies obtained by splitting them into two (disruption of disulfide bonds between the two heavy chains) [62], the objective being to increase the density of the recognition sites. Concerning the OSs, the binding density of the aptamers must be facilitated by the possibility to introduce a chemical function during their synthesis according to the nature of the solid-phase selected for their immobilization. In addition to the introduction of this functionality, it was also shown that the length of the spacer affects the binding density. Indeed, when studying the immobilization of aptamers, specific to OTA, on CNBr-activated sepharose, a higher capacity was obtained using an aptamer linked with a C12 spacer arm to the amino-group than using a C6 spacer arm [25]. In return, the immobilization of an aptamer through the 30 -end side and from the 50 -end side gave rise to similar capacities [25]. Values of capacity found in the literature for target molecule on OSs available as SPE cartridges are quite homogeneous and are in the range 0.26e69 nmoL/g of OS [4]. Such capacity values are associated with binding density values in the range

Aptamer-based and immunosorbents

173

19%e37% on Sepharose beads, a binding density of active aptamers of 68% being recently reported [17]. Those capacity values for OSs are very close to those obtained with immunosorbents (from 4 to 93.6 nmoL/g of sorbent, e.g., immunosorbents specific to pesticides) [93]. While it is sufficient to adapt the quantity of IS/OS introduced into an extraction cartridge to ensure sufficient capacity, the miniaturization of extraction devices has led to the evaluation and comparison of new immobilization approaches. As an example, as the grafting of antibodies directly immobilized on the inner wall of a capillary was low, the use of NP-coated capillary was proposed and has allowed to reach a bonding density to increase the sorbent capacity by a factor 5 [94]. The same group recently proposed to modify first the capillary surface by NPs further functionalized with antibodies that were immobilized in an orientated way (immobilization of the pAbs through the oxidized carbohydrate chain located on their Fc part). The orientation of the antibodies allowed to improve the capacity of the immunosorbent by a factor three compared to a random immobilization and the use of NPs by a factor 1.5 [95]. To improve the capacity of such miniaturized OSs, the introduction of NPs grafted by aptamers and further immobilized on monolith was also proposed [29,32].

6.6

Contribution in the selectivity of the OS/IS and control of nonspecific interactions

Nonspecific interactions (NSIs) can occur between the analyte and the sorbent used for the immobilization of aptamers/antibodies and those macromolecules too. These NSIs can contribute to the unexpected retention of interfering compounds present in the sample, thus affecting the selectivity of the procedure. Agarose gel, such as Sepharose and hydrophilic monoliths have been often selected as solid-phase to reduce NSIs as much as possible, i.e., mainly hydrophobic and/or electrostatic interactions favored in aqueous media. The comparison of the retention of the target analyte on the IS/OS with the nongrafted solid-support, named control sorbent [17,19,20,24e27,29e32,44,69,70,72e75,83], or grafted with another antibody/oligonucleotide sequence, having no affinity for the target [17,19,20,27,32,33,39e41,44,48,69,83], is now often reported to evaluate the contribution of these NSIs. The use of these sorbents in parallel to IS/OS is particularly helpful for optimizing the washing step and then obtaining an optimal selectivity [33,57]. At least the evaluation of the selectivity can be achieved by controlling the lack of retention of compounds often present in the studied matrices and not recognized by antibodies/aptamers [25,30e33,41,45,46,48e51,53,54,71,73,84].

6.7

Specificity toward structural analogs

Oligosorbents and immunosorbents largely differ by their ability to retain target structural analogs. Indeed, antibodies may present a broad cross-reactivity, particularly

174

Solid-Phase Extraction

when they have been produced for small molecules, thus allowing the trapping of a structural group of compounds. This is illustrated by the commercialization of ISs during the last decade for the clean-up of samples for the analysis of classes of mycotoxins (aflatoxins, ochratoxins, fumonisins, etc.) in food, of the class of veterinary drugs (clenbuterol and analogs) and of drugs of abuse (LSD and its metabolites, etc). The commercialization of ISs containing several antibodies specific for several toxins for their simultaneous trapping is now also proposed to expand this specific retention to compounds from different classes present in the same samples on a single cartridge. In return, the ability of aptamers to retain structural analogs can be poor. As an example, OTA aptamers present 100-fold less affinity for Ochratoxin B than for OTA, a difference of only 3-fold being observed for antiOTA monoclonal antibodies [22]. Moreover, a slight change in a sequence leads to loss of molecular recognition as it was shown by the loss of cocaine retention on an OS prepared with a DNA sequence similar to the DNA sequence of the cocaine aptamer but presenting only one mutation [20]. Concerning theophylline aptamers, they have no affinity for caffeine despite the structural analogy of these molecules that only differ by a methyl group [96]. However, in this last case, the SELEX procedure was carried out in order to remove oligonucleotides that could present an affinity for caffeine, using a counter selection process, thus showing that the affinity of an aptamer to recognize some structural analogs depends strongly on the SELEX process. Indeed, the SELEX procedure can also be adapted in order to select aptamers showing an affinity for different targets by using them alternatively during the selection. This shows the possibility offered by the SELEX technology when preparing aptamers to control their specificity that cannot be achieved with antibodies.

6.8

Reusability, regeneration

The reuse of ISs is not recommended by the suppliers of commercial immunoaffinity cartridges. However, many studies show that their regeneration is possible, which is interesting considering their cost. Indeed, despite their susceptibility to irreversible denaturation in some conditions, numerous studies have shown the possibility to reuse ISs after their storage at 4 C in a PBS solution, which is close to the physiological conditions and in the presence of an antimicrobial agent, such as sodium azide. This possible regeneration was confirmed recently by a detailed study related to the reusability of commercial ISs dedicated to mycotoxin analysis [97]. In the same way, despite their sensitivity to nuclease, OSs can be stored at 4 C in a buffer solution, whose composition is close to the binding buffer, and also containing sodium azide. Their reusability can be explained by their high stability to low and high temperature, broad pH range, and salt, these different parameters affecting the binding of the target without causing irreversible denaturation. This can be explained by the stability of the phosphodiester bond and by the possibility to improve their stability by chemical modification [98]. In addition to their less expensive preparation, aptamers have short renaturation times, and can, therefore, be

Aptamer-based and immunosorbents

175

reused after a few minutes in a buffer a few minutes against 24 or 48 h for antibodies. Thus, the reusability of OS-SPE cartridges has been demonstrated, as for IS, by their successive application, 5 times [23] and even 15 times [25], to the purification of cereal extracts. The reusability also depends on the nature of the bonding, noncovalent bonding being less stable than covalent bonding as previously mentioned. It was also reported that the use of orientated immobilization of antibodies also gave rise to higher stability of the IS compared to random immobilization [95]. Of course, the stability of these sorbents depends on the number of applied real samples. The reusability of OS and IS was also demonstrated for miniaturized devices. As an example, an OS-SPME fiber was applied up to 20 times to plasma samples [83]. For IS-SPME, Yao et al. mentioned that the reusability was evaluated once every 3 days for 45 days by column capacity determination showing a loss of the capacity by a factor two during this period [82]. This is in good agreement with another study mentioning that only 58% of the binding capacity remained after 10 uses [81]. Similar results were reported for IS monoliths that were reused up to 50 times without observing any loss of trapping efficiency [60]. In return, some authors observed losses after 5 [99,100] or even 3 [101] uses on pure samples. This loss of performance after 3 uses was observed on 6 monoliths (whose performance was similar for the first uses) confirming this loss of performance. The results of the first use nevertheless illustrate the repeatability of the monolith preparation also reported by other groups [57,60]. Concerning this last point, it can be considered that the commercialization of Sepharose based-ISs illustrates the reproducibility of their preparation. Concerning other formats, such as miniaturized ones, the reproducibility of the synthesis of the monolith and of the grafting of antibodies or of aptamers was already demonstrated [33,57].

6.9

Conclusion

The potential of using immunosorbents for the selective extraction of molecules from complex samples is no longer demonstrated. In order to reduce the development cost of these selective supports, it seems interesting to replace antibodies by aptamers, which allows the developing of equally varied extraction tools with similar potential in terms of extraction yields and selectivity. However, it is important to note some properties that make them different, such as their ability to recognize different analogs, as a result of an immunization choice for some or as a result of selection criteria for others. The possibility of easily introducing certain chemical functions to assist in their grafting can increase the interest of aptamers. However, it is still important to continue work on identifying aptamers sequences of sufficient affinity, of the nanomolar order as is generally obtained with antibodies, to allow the development of supports with high retention potential.

176

Solid-Phase Extraction

List of abbreviations AFP APTES BB APTMS CEA CNBr DADPA dSPE EDC EDMA FITC GMM IS g-MAPS MOF MPTMS MTMS NHS NP OS PANCMA PBS PDDA PDITC PDMS PEGDA PETA POSS PP SBSE SPE SPME SSMCC TEG TFME TMOS VTMS

a-fetoprotein (3-aminopropyl)-triethoxysilane binding buffer aminopropyltrimethoxysilane carcinoembryonic antigen cyanogen bromide diaminodipropylamine dispersive solid-phase extraction ethyl-dimethylaminopropyl-carbodiimide ethylene dimethacrylate fluorescein isothiocyanate GMA: glycidylmethacrylate glycerylmethacrylate; GTD: glutaraldehyde immunosorbent g-methylmethacrylate trimethoxysilane metal organic framework mercaptopropyltrimethoxysilane methyltrimethoxysilane N-hydroxysuccinimide nanoparticle oligosorbents poly(acrylonitrile-co-maleic acid) phosphate buffer saline poly(diallyldimethylammonium) phenylene diisocyanate polydimethylsiloxane polyethyleneglycoldiacrylate pentaerythritoltriacrylate PMMA: polymethylmethacrylate polyhedral oligomeric silsesquioxane protein precipitation stir bar sorptive extraction solid-phase extraction solid-phase microextraction sulfosuccinimidyl-4-(N-maleimido-methyl) cyclohexane-1 ecarboxylate triethyleneglycol TEOS: tetraethyl orthosilicate thin-film micro-extraction tetratrimethoxysilane vinyltrimethoxysilane

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Leon Fuks, Irena Herdzik-Koniecko Centre of Radiochemistry and Nuclear Chemistry, Institute of Nuclear Chemistry and Technology, Warszawa, Poland

7.1 7.1.1

Introduction Metals

A stair-stepped line, starting at aluminum (Al; Z ¼ 13) and going down to polonium (Po; Z ¼ 84), separates into two groups of elements in the periodic table: metals and nonmetals. The only exceptions within the group of metals are germanium (Ge; Z ¼ 32) and antimony (Sb; Z ¼ 51). In the following scheme, border-line elements between metals and nonmetals are presented in green (Scheme 7.1) To characterize metals in short, one must state, that: U U U U U U

metals are solids with the exception of mercury (Hg), which is a liquid they are shiny they are good conductors of electricity and heat they are ductile (i.e., they can be drawn into thin wires) they are malleable (which means that they can be easily flattened into thin sheets) all metals have a tendency to lose a number of electrons depending on their position in the periodic chart, and this feature determines their chemical properties.

Scheme 7.1 Metals and nonmetals in the periodic table of elements. Solid-Phase Extraction. https://doi.org/10.1016/B978-0-12-816906-3.00007-8 Copyright © 2020 Elsevier Inc. All rights reserved.

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The whole group of metals may be divided into several subgroups, i.e., the heavy metals, transition metals (including the rare earth and the transuranic), alkaline earth, and the alkali metals. Below, we present the characteristics of each group in a condensed form: U The term heavy metal is applied for the metals which are comparatively of high density (it is assumed as more than 5 g/cm3) and less reactive than lighter metals. Physical and chemical characterization of heavy metals needs to be made with great care, as the group is not always strictly defined. The most popular definition was proposed by S.J. Hawkes, which says that heavy metals comprise a block of all the metals in Groups 3 to 16 that are in periods four and greater [1]. Typically, this group contains: aluminum (Al), antimony (Sb), arsenic (As), barium (Ba), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), gold (Au), iron (Fe), lithium (Li), nickel (Ni), lead (Pb), manganese (Mn), mercury (Hg), molybdenum (Mo), platinum (Pt), rubidium (Rb), silver (Ag), scandium (Sc), selenium (Se), strontium (Sr), tin (Sn), titanium (Ti), tungsten (W), uranium (U), vanadium (V) and zinc (Zn). Heavy metals are natural components of the Earth’s crust, but anthropogenic activities, such as the mining of heavy metals, their industrial production and usage, agricultural use, and use in household applications are the main sources of heavy metal pollution. U According to the IUPAC nomenclature, any element with a partially filled d-electron subshell is called a transition metal. The metals are placed in groups 3 through 12 in the periodic table. In turn, the block of electron elements (lanthanides and actinides, situated below the main body of the periodic table) are called the inner transition metals. Transition metals exhibit a wide range of oxidation states. Complex formation by transition metals is crucial to the separation of these metals. According to the broader definition, coinage metals (Cu(II), Ag(II), and Au(III)) are generally considered transition elements. The elements in group 12, in turn, are not typically considered as transition metals, although chemically they frequently resemble the transition elements. U Beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra) are all called as the alkaline earth metalsdchemical elements in the s-block of the periodic table with very similar properties. All of the alkaline earth metals are commercially valuable and do not occur as a free metal in nature as they are very reactive. U The alkali metals are the elements placed in the first group of the periodic table. Namely, lithium (Li), sodium(Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). Alkali metals are even more reactive than alkaline earth metals, and none of them occur as a free metal. Many of the alkali metal compounds (NaCl, KCl, Na2CO3, NaOH) are commercially important, so the process of their separation is the subject of many studies.

A large number of different technologies have been developed for sequestering metals from wastewaters and for separation of individual metals. These methods are based on the processes of coagulation, filtration, ion exchange, solvent extraction, foam flotation, activated sludge, aerobic and anaerobic treatment, microbial reduction, electrolysis and, ultimately, the process of adsorption [2]. Adsorption is as effective as the other techniques but does not possess certain of their limitations. The most important of these is the formation of a large amount of sludge, low efficiency, sensitivity of the operating conditions to various physical or chemical factors, high cost of operation, and difficult waste disposal. In addition, the adsorption technology is easy to adapt to

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specific conditions, offers high-purity of both the resulting solution and adsorbate and the adsorbent can be regenerated [3]. According to the Scopus database, during the last 10 years, more than 4500 reports were published in the field of the heavy metal sorption.

7.1.2

Basic terms used in sorption science and technology

Generally speaking, sorption is a physical and/or chemical process, leading to binding of certain components of the liquid or gaseous mixtures (sorbates) with the immiscible solid matter (sorbent). The common term sorption includes three similar, but different, phenomena: U Absorption, which occurs when the sorbate enters a sorbent material. In the course of this process, molecules of the sorbate are dissolved or diffused in the absorbent and form a solution. Once dissolved, the molecules cannot be easily separated from the absorbent. U Term adsorption describes the process when the molecules are attached to the surface of the adsorbent and can be easily removed from it. U Ion exchange process consists of an exchange of ions between two electrolytes or between a solution containing the sorbate and the chemical groups present on the surface of the sorbent. The so-called sorbing groups should show the ability to release ions of the same sign (electric charge) as the ions of the sorbate.

As the list of basic terms used in sorption science and technology can be found in many publications [e.g., Refs. [4,5]], here we present them only in the visual form of Fig. 7.1.

Intraparticle adsorbate Monolayer adsorbate Ion exchange Multilayer adsorbate Boundary layer

Bulk transport

Adsorptive

Film

Pore

Bulk Film transport

Absorbate

Figure 7.1 Some basic terms used in sorption science and technology.

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Figure 7.2 Two types of the metal-selective sorption processes: batch mode (left), and dynamic mode (right).

The sorption of metals with the aim of decontamination of aqueous solutions and possibly further re-use of the metals may be done in several ways (see, Fig. 7.2): U Batch (static) method: due to the extensive contact between the immiscible phases, the sorbate migrates from the solution to the sorbent. Afterward, both immiscible phases may be separated either by gravity (sedimentation) or by centrifugation. Certain sorbents, with magnetic properties, can be separated by an external magnetic field. For sorbent recovery and to obtain the sorbate in a pure form, some supplementary operations are necessary (e.g., desorption). U Column (flow) method, sometimes called column chromatography, is preferred for the separation of mixtures of several substances. In this technique, a liquid or gaseous solution is forced to flow through a tube filled with a thoroughly ground solid (sorbent). If necessary, the solid column filling may be coated with a liquid phase. Dissolved components of the solution may be separated because they move along the column at different rates, which depend on the strength of their interaction with the stationary phase.

Results of the sorption process are usually expressed in terms of the equilibrium partition constant Kp, which is defined as the ratio of the concentrations of sorbate in both heterogeneous phases in equilibrium with each other. It is related to the specific adsorption capacity of the dry sorbent (qM, mg$g1), which depends mainly on the solution pH, initial metal concentration, presence or absence of other solution components (often with competitive affinity for the sorbent) and temperature of the process, as well as on dimension of the sorbent particles and the pretreatment method of the sorbent [6e8]. The last two factors may significantly increase the porosity of the sorbent surface, a crucial parameter for determining sorption efficiency. For the sorption of metals the most decisive factor, however, is the speciation of the metal ions compared with the protonation of the binding sites of the sorbent. Both these features depend predominantly on the acidity and composition of the purified solution.

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Figure 7.3 Examples of sorption isotherm curves. Sorbent B has better sorption properties (higher q) than sorbent A at lower equilibrium concentrations but worse (lower q) at higher concentrations.

The functional relationship between the sorbate amount and the sorbent in the equilibrium state is called the sorption isotherm [9] as shown in Fig. 7.3. In turn, the kinetics of the overall sorption process is typically represented as a sum of several consecutive stages. In the first, metal ions are transported to the sorbent surface controlled by the diffusion process. In the next, their diffusion on the outer surface of the sorbent (called as the film diffusion) is followed by penetration inside the pores (pore diffusion). At last, on the inner surface of the pores, metal ions interact with the unoccupied active sites of the sorbent. So, the cumulative rate of the sorption is called a pseudo model of the rate [10]. From the chemical point of view, the affinity of the metal ion for the binding sites of the sorbent, as well as the selectivity of the separation in the case of the multimetal solutions, is typically expressed in terms of the Pearson’s theory of hard and soft acids and bases [11]. It means that the hard acids (i.e., metals of low polarizability, such as Mg(II), La(III), Fe(III), Cr(III), U(VI), V(IV) and so on) favor bonding with hard bases (i.e., groups containing the electron donor atoms) ranked in the following order: N [ P, O [ S, F [ Cl. On the contrary, soft acids (e.g., Ag(I), Cd(II), Hg(II), Au(III), or Sn(II)) have a preference to bind with soft bases (i.e., P [ N, S [ O, I [ F). A significant number of hard and soft borderline metals have intermediate binding properties, and are generally categorized with the IrvingeWilliams series for their stability constants: Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) < Zn(II). Therefore, the affinity of alginates for metals forms the following series: Pb(II) > Cu(II) > Zn(II) > Co(II) > Ni(II) > Mn(II) > Fe(II) [12,13]. On the other hand, the selectivity of metal separations on chitosan, due to several types of binding groups with different electron donor atoms (see, Fig. 7.4), do not form a logical series with respect to either ionic size or hardness of the metal cations [14]. Therefore, even today,

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Figure 7.4 Selected binding types occurring in the adsorption of divalent metal compounds by sorbents with different binding groups.

there is no uniform theory that allows the prediction of the sorption results in different systems. Consequently, our present-day knowledge of sorption characteristics is based mainly on tabularized experimental data.

7.2

Sorbents for sequestering metal ions from aqueous solutions

Sorbents are generally fine-grained solids, the particles of which show a large surface area and, as a result, noticeable sorption properties. Depending on their origin, sorbents may be divided into two main groups: natural and synthetic materials. Furthermore, within these groups, sorbents can be classified as mineral sorbents (also called inorganic materials), carbons (activated carbons, carbon nanostructures, fullerenes, and graphenes) and organic sorbents (usually obtained from so-called biomass). For heavy metals, the classification of Fig. 7.5 is proposed. Natural mineral sorbents are produced from the crushed rocks, thermally treated if necessary. The thermal treatment of the raw, granular materials is to increase the surface area and porosity of the sorbent. Particles of 0.1e1 mm are considered small granules, while these of 1e3 mm are treated as rough. Depending on the particle size, mineral sorbents are used for internal use (small grains) or external practice (large grains). Rough-grained materials are more massive and more chemically resistant than finely ground materials. Natural inorganic sorbents include clay, sand, and even volcanic ashes. Natural organic sorbents are produced from either locally available raw biomass, e.g., from the tree cortex or sawdust, peat, lichens, etc., or any natural product that contains carbon, mainly, industrial, agricultural, or pharmaceutical wastes. Typically, they indicate better sorption properties than mineral wastes, but also lower stability in contact with aggressive chemicals or from mechanical impact. Thus, they are not

Metal-selective sorbents

191

Figure 7.5 Classification of sorbents used for metal recovery or concentration.

commonly used in the removal of metals from aggressive fluids such as acids, alkali, and oxidizing substances. Synthetic sorbents represent the man-made materials that generally resemble plastics and are designed to adsorb harmful or reusable constituents from solution. Some other synthetic sorbents, crosslinked polymers or rubber materials, absorb liquids into their solid matrix, and as a result of this process, the sorbent swells. Synthetic sorbents, usually, are more efficient than natural sorbents [15], which should be activated in order to acquire the appropriate physical, chemical, catalytic, and adsorptive properties. Most of the commercial sorbents produced on a large scale, e.g., silica gels, zeolites, aluminosilicate minerals, activated carbons, cyanoferrate complexes with different metals, and others [e.g., Refs. [16e18]] with numerous advantages, demonstrate certain limitations. Among these, one should mention an unfavorable surface area

192

Solid-Phase Extraction

Figure 7.6 Basic units of the biopolymers: chitosan (left) and alginic acid (right).

and morphology, insufficient pore size distribution, as well as the type of the functional groups attached to the adsorbent surface [19]. In addition, the cost of these materials is generally high [20]. Therefore, there is a need to find novel sorbents, in order to decrease material costs for sequestering metals or modifying the existing sorbents with intent to improve their sorption properties. These novel sorbents often do not occur naturally, but as a rule, are relatively inexpensive [21]. Contemporary database for metal sorption using different sorbents contain a large number of records, and new information is added continuously. In addition, some sorbents are identified only by their name and lack important chemical information. For example, chitosan of different origin differs significantly in the degree of deacetylation or the alginates differ by the ratio of the guluronic and the mannuronic acids (see Fig. 7.6). In spite of the large amount of experimentally derived information, their structural diversity does not allow even the most general conclusions to be drawn.

Table 7.1 Methods of modification of the sorbents [22]. Type of the modification Physical modification

Chemical modification

Examples

By addition of the components which improve the physical properties

Application of such additives as: U plasticizers, U fillers

Action of the physical factors

Application of such impulsions as: U thermal energy (thermal treatment), U ultrasound, U electric field U magnetic field U ionizing radiation

Modification of the existing sorbent

Synthesis of e.g.,: U phenyltrimethylammonium (PTMA)- or benzyltrimethylammonium (BTMA)-bentonites U Crosslinking the polymers

Modification in the course of the synthesis

U Sulfation of sodium alginate prior to its gelation U Acidic treatment or impregnation of the activated carbon

Metal-selective sorbents

193

Modification of sorbents allows a number of their physicochemical and/or process properties to be corrected. Such methods are generally more efficient than the synthesis of novel materials. Typically, it can be performed in two ways: chemical and/or physical modification (see Table 7.1). Physical modification is usually easier and less expensive compared with chemical modification. However, the mechanism of the process is sometimes complex. It is also known as a structural modification, because the action of certain physical factors results in, mainly, modification of the molecular structure of the material. Chemical modification leads to improved properties by controlled alteration of their molecular composition. Common reactions are used to introduce new functional groups, to convert specific functional groups to a different form (e.g., esterification, deacetylation), oxidation or reduction reactions, grafting, crosslinking of polymer chains or their degradation. These modifications may be introduced during synthesis or are made to an existing sorbent.

7.2.1

Biomass, industrial and agricultural by-products and wastes

Removal of toxic metals or recovery of valuable metals from aqueous solutions using biosorbents, products obtained from plants, living organisms, or from the industrial and agricultural by-products, is the driving force in the search for novel and environment-friendly sorbents [23]. The initial challenge is the selection of the most promising types of biomass from the extremely large group of locally available and inexpensive types of biomass [23]. To facilitate this process, the biosorption capacity of various biomasses is generally determined [24e26]. However, the application of biosorbents for metal recovery and separation rarely extends beyond the laboratory scale at present. In addition, the isolation of the metal-loaded biosorbent from the solution is often difficult. Recently, the synthesis of magnetic composite biosorbents has been initiated [27]. Biosorbents can be assigned to one of the following groups: bacteria, fungi, algae, industrial wastes, agricultural wastes, natural residues, and other biomaterials [23]. There are many review papers on this topic, so in Table 7.2, we present only an extract of the data concerning the sorption of lead(II) by popular plant materials [28]. Table 7.2 Pb(II) sorption capacity of some plant-derived low-cost biosorbents; pH 5 [28]. Biomass type

qM (mg.gL1)

Biomass type

qM (mg.gL1)

Grass clippings

28.2

Peepul leaves

107.5

Tea leaves

33.1

Mango leaves

30.0

Banana peels

63.6

Rice husk

29.5

Teak sawdust

38.8

Rice stem

45.1

Peanut hulls

56.4

Coir fibers

48.2

194

Solid-Phase Extraction

Table 7.3 Values of qmax (mg,g1) for the main monometallic systems for different types of algae [30]. Ni(II)

Cu(II)

Algae

Zn(II) (mg$g

L1

Cd(II)

Pb(II)

)

Codium vermilara

13.2

16.9

23.8

21.8

63.3

Spirogyra insignis

17.5

19.3

21.1

22.9

51.5

Asparagopsis armata

17.1

21.3

21.6

32.3

63.7

Chondrus crispus

37.2

40.5

45.7

75.2

204.1

Ascophyllum nodosum

43.3

58.8

42.0

87.7

178.6

Fucus spiralis

50.0

70.9

53.2

114.9

204.1

Table 7.4 Sorption capacity of some metals on unmodified biosorbent (a mixture of tea waste, maple leaves and mandarin peel) versus chemically modified [31]. Cd (mg.gL1)

Cu (mg.gL1)

Pb (mg.gL1)

Zn (mg.gL1)

A mixture of tea waste, maple leaves and Mandarin peel

31.73

41.06

76.25

26.63

Raw material acid treated in the elevated temperature

69.56

127.70

245.20

70.55

At the same time it is worth mentioning that the sorption properties studied for the same sorbent, but of different origin, may differ significantly from each other. One such example is presented in Table 7.3 [29,30]. Sorption properties of most biosorbents may be improved by simple methods, such as boiling with concentrated mineral acids or bases. One such example is illustrated by Table 7.4 [31]. To improve their activity for different metals, biosorbents may be functionalized by binding different chemical compounds. Two examples employing chitosan are illustrated in Tables 7.5 and 7.6 [32].

7.2.2

Activated carbons

Activated carbon (also referred to as activated charcoal or active carbon) is one of the oldest and widely used forms of carbon for the removal of contaminants for wastewater treatment. Due to the high porosity, activated carbons have surface area surpassing 3000 m2/g. This high porosity facilitates the sorption of metals and dies. The material

Metal-selective sorbents

195

Table 7.5 Sorption capacity for Cu(II) by chitosan and chitosan-based. Materials

Sorption capacity mg,gL1

Chitosan

80.71

Chitosan-crosslinked GLA

59.67

Chitosan-crosslinked ECH

62.47

Chitosan-crosslinked EGDE

45.94

Copper(II)-imprinted chitosan

201.66

Nonimprinted chitosan without template

89.51

Microspheres crosslinked carboxyl-grafted chitosan

5.01 6

N,O-carboxymethyl-chitosan

162.5

Xanthate-modified magnetic chitosan

34.5

Carboxymethylated chitosan-bound Fe3O4 nanoparticles

21.5

Chitosan-tripolyphosphate beads

26.06

Chitosan-tripolyphosphate chelating resin

200 3.15

Gel beads crosslinked magnetic chitosaneisatin Schiffs base resin (CSIS)

103.16

Chitosan electrospun nanofiber mats

485.44

Porous chitosan functionalized with histidine

127e190

Different chitosanezeolite composites

14.75e51.32

Bacillus subtilis immobilized on chitosan

100.7 1

Table 7.6 Chitosan and derivatives of metal affinity order. Sorbent types

Affinity or selectivity order

Magnetic chitosan microspheres modified with thiourea

Mg,g1: 625.2 (Hg2þ), 66.7 (Cu2þ), 15.3 (Ni2þ)

Xanthate-modified magnetic chitosan beads

Pb(II) > Cu(II) > Zn(II)

Chitosan film

Cu(II) > Hg(II) > Zn(II) > Cd(II) > Ni(II) >Co(II) z Ca(II) and Eu(III) PNd(III) > Cr(III) z Pr(III).

Crown ether grafted chitosan

Pd (II) or Ag(I) >Pb(II) and Cr(III)

EDTAechitosan

2þ Cu2þ z MoO2þ 2 > Ni > VO2 2þ 2þ 3þ Zn  Co > Al

þ

>

196

Solid-Phase Extraction

Figure 7.7 Left: Type of typical carbon sorbents (A) graphite (B) diamond (C) amorphous carbon (D) nanotube (E) fullerene and (F) graphene. Right: Main oxygen-containing groups on the surface of activated carbon suitable for modification: carboxyl, carbonyl, and phenol.

obtained from coal is generally referred to as activated coal, while that obtained from coke as activated coke. Different functional groups exist on the surface of the sorbent (right part of Fig. 7.7), but further chemical treatment enhances its adsorption properties. As an example, the maximum sorption capacity for lead(II) ions was found to be about 19 mg g1 for activated carbons. Acid-base treatment of these sorbents was found to double the sorption capacity. Immobilization of baker’s yeast on the surface of activated carbon sorbents generally improves the sorption capacity of lead, up to 120 mg g1 [33]. Metal adsorption on activated carbons takes place mainly in micropores and to a small extent in the mesopores. Macropores, act only as flow-channels for the adsorbate transported toward the smaller pores. The method of activation and nature of precursors for the production of active carbons strongly influences the type of surface functional groups and the pore structure of the activated carbon. Various methods, such as acid or base treatment, surface impregnation, ozone or surfactant treatment, plasma or microwave treatment, have been used. These methods have been reviewed, e.g., in Ref. [34]. Several methods have been proposed for the synthesis of graphene (G) and its modification to graphene oxide (GO) [35,36]. GO contains numerous functional groups containing oxygen atoms, especially epoxy, hydroxyl, and carboxylic acid groups [37]. Presence of these groups determines the hydrophilicity and the extent of the negative charge density of GO. Several structural models of GO have been already proposed, all of which show the existence of regular lattices formed by the repeating units, for example, Fig. 7.8 [38]. GO is currently the most actively researched carbon sorbent for the sorption of metal ions [39e41]. A few representative examples are summarized in Table 7.7 [42]. Carbon nanotubes are not as widely used, and a few representative examples are summarized in Table 7.8 [3].

Metal-selective sorbents

HO2C

CO2H

CO2

CO2H

CO2

CO2H

HO2C HO2C

CO2H

HO2C

= Metal ion

O2C

CO2

O2C O2C

CO2

O2C CO2H

CO2

Figure 7.8 Structure of the graphene oxide GO (left) and its interaction with heavy metal cations (right).

197

198

Solid-Phase Extraction

Table 7.7 Metal sorption capacities on the GO and some modified GO materials. Selected data cited from Ref. [42]. Cr(VI)

Co(II)

Cu(II)

Adsorbent

Zn(II)

(mg$g

L1

Graphene oxide (GO)

68.2

46.6

GO/Fe3O4

13.0e22.7

Cd(II)

Pb(II)

14.9

367

) 30.1

106.3

GO-EDTA

479

GO-Chitosan

99

rGO-ED

5

Table 7.8 Maximum sorption capacities of metal ions on the carbon nanotubes [3]. Cr(VI)

Co(II)

Ni(II)

Adsorbenta

Cu(II)

Cd(II)

(mg$gL1)

CNTs

102.04

MWCNTs

18.08

Acidified MWCNTs

85

Oxidized CNTs Oxidized MWCNTs

49.26 4.262

25.7

CNTs immobilized by calcium alginate MWCNTs/iron oxide composite a

Pb(II)

67.9 9.18

CNTs - carbon nanotubes; MWCNTs - multiwalled carbon nanotubes.

7.2.3

Mineral adsorbents

A number of mineral sorbents are commonly used for metal separation because of their advantages compared with biomaterials. As they have a high specific gravity, they are also known as sinking sorbents. The most important qualities are their incombustibility, chemical inertness, are relatively inexpensive, and widely available. Generally, prior to their use as sorbents minerals, they are ground to a particle size of several nm to several mm (usually, 3 mm) and a bulk density of about 0.45e0.90 kg dm3. These sorbents are not often used for the removal of metals from water because of their low buoyancy and absorbability comparable with polymers or natural organic sorbents. However, after modification with organic compounds, they are suitable for water media.

Metal-selective sorbents

7.2.3.1

199

Clay minerals, zeolites, silica minerals

Natural layered aluminosilicates, e.g., the bentonite or vermiculite, are important raw materials for the production of mineral sorbents. They are widely available with many methods for their modification already developed. These procedures allow the design sorbents with specific properties for the selective separation of metals. The most commonly known representatives of the clay minerals are kaolinite, montmorillonite, and sepiolite. The presence of hydroxyl groups and weak electrostatic interactions between layers/sheets and the exchangeable cations determine their sorption properties. Zeolites are natural minerals with others obtained synthetically. Natural zeolites were formed as the result of weathering of volcanic rocks. At present, about 40 groups of zeolites are known, and most of them have been evaluated as sorbents for metals. Silica minerals are a partially dehydrated polymeric form of colloidal silicic acid with the formula SiO2$nH2O. This amorphous material comprises spherical particles 2e20 nm in diameter, which may aggregate to form adsorbents with pore diameters of 6e25 nm. The silica sorbents group includes rocks (siliceous earth, diatomaceous earth, diatomites) and perlite minerals. Most are mineraloids (e.g., opal and chalcedony) with some having a noticeable crystal structure (e.g., cristobalite, quartz, clay minerals, and carbonates). Silica sand is widely used in sorbent production for water purification and metal separations because of its favorable adsorption properties [43]. Aluminum silicates form a group of minerals in which silicon is present in the form of tetrahedral [SiO4]4- connected via oxygen forming structurally complex two- and three-dimensional systems. They can be distinguished depending on the number of layers present, i.e., the 1:1 or 1:2 minerals. Sometimes, we also find amorphous forms of aluminosilicates, the so-called allophane Al2,SiO2,nH2O. Zeolites are crystalline aluminosilicates of the alkali metals and different
divalent metals. Their basic structural units are tetrahedral silicon dioxide SiO4 4 . Tetrahedrons, linked by common oxygen atoms, form a three-dimensional structure containing channels with diameters between 0.3 and 3 nm, and are known as molecular sieves. Some of the silicon atoms can be replaced by aluminum atoms of lower valence with the formation of a negative charge that determines their sorption properties. Representative structures of both mineral types are shown in Fig. 7.9. The hydroxyl groups offer surface sites ready for chemisorption of metal ions [44,45].

Figure 7.9 Structure of aluminosilicate (left) and zeolites of the A and X type (right).

200

Table 7.9 The removal capacity of various natural clays and clay minerals toward various heavy metal ions [46]. Ag(I)

Cr(III)

Cr(VI)

Co(II)

Adsorbent

Cu(II)

Ni(II)

(mg$g

Zn(II)

Cd(II)

Hg(II)

)

Palygorskite

104.3

Montmorillonite

21.10

30.70

31.10

Kaolinite

11.52

Cankiri bentonite

80.64

Illitic clay Expanded perlite

Pb(II)

L1

39.9

52.5

8.46

1.95

Brazilian smectite

0.35

60.32

Brazilian sepiolite Brazilian kaolinite Clinoptilolite

80.93 2.4

1.5

3.8

0.9

2.7

3.7

6.0

Chabazite [47]

3.6

5.8

5.1

4.5

5.5

6.7

6.0

6.80

11.5

Kaolinite Palygorskite

7.10 8.88

Solid-Phase Extraction

Clinoptilolite [47]

25.01

Dolomite

19.69

Perlite

8.91

Apatite

82.88

Sericite Kaolinite Sericite Cankiri bentonite Natural zeolite [29]*

44,00

Metal-selective sorbents

Diatomite

3.01 1.67

4.69

44.84

4.69

1.12

1.32

201

202

Solid-Phase Extraction

The sorption behavior of natural zeolites for metal ions has been intensively studied, and there are a number of excellent reviews, e.g., Ref. [48]. As an example, cadmium(II), copper(II), nickel(II), lead(II) and zinc(II) are sorbed on clinoptilolite [49]. The zeolite sorbs about 32%, 75%, 28%, 99%, and 59% of each metal, respectively, from solutions containing metals in the concentration range 50e300 mg g1. The selectivity sequence of the metals is Pb(II) > Cu(II) > Zn(II) > Cd(II) > Ni(II). Certain examples of the natural mineral sorbents are shown in Table 7.9.

7.2.3.2

Modified mineral adsorbents

Many attempts have been made to improve the selectivity of mineral sorbents through chemical modification. Generally, these modified sorbents have been prepared by thermal treatment or functionalization of the mineral’s surface with organic compounds. Physical modification of the sorbents is possible by calcination (e.g., diatomite, perlite, or vermiculite treated at a temperature above 1000 C). Although production costs are relatively high an advantage of these materials is their facile desorption of the adsorbed metals [50]. In recent years, several methods of surface modification of mineral sorbents by different surfactants have been described [51e53]. This method is associated with either changing the character of the surface from hydrophilic to hydrophobic or the creation of metal chelating centers. The process of chemical modification of mineral sorbents increases the cost of the final product and organo-mineral sorbents are commonly used at the industrial scale. Due to the potential toxicity risk related to the use of frequently harmful modifiers, the disposal of metal-loaded sorbents can be a problem [54]. Certain examples of modified sorbents are shown in Table 7.10.

7.2.4

Synthetic sorbents

Synthetic sorbents are more widely utilized materials for the recovery, removal, or concentration of heavy metals by solid-phase extraction (SPE) method than natural materials. The development of the production of synthetic sorbents for better sorptive properties of metals is still on-going. Among synthetic sorbents, three basic categories can be distinguished, namely, inorganic sorbents (e.g., metal oxides), organic sorbents (e.g., polymers), and hybrid sorbents (mixtures of inorganic-organic sorbents). To the synthetic inorganic adsorbents belong silica gels, metal oxides (such as aluminum oxides, magnesium oxides, ferric oxides, manganese oxides, titanium oxides, cerium oxides), zeolites, aluminosilicate minerals, activated carbons, cyanoferrate complexes with different metals and others. Synthetic organic sorbents are a group of polymers with ion exchange or chelating functional groups. They have large surface areas with a high uptake capacity, good pore structures, and efficiently and selectively adsorb metals from aqueous solutions. Solid-phase extraction processes with synthetic adsorbents allow reducing the amount of solvent and safer procedures compared with liquid-liquid extraction methods. A further advantage of synthetic adsorbents is their stability in both acidic and alkaline solutions and in organic solvents. Synthetic sorbents can be easily regenerated for further use. The attrition resistance of synthetic sorbents is also better than that for natural materials allowing their use in a larger

Metal-selective sorbents

203

Table 7.10 Sorption of metals on the modified minerals (selected data). Mn(II)

Ni(II)

Cu(II)

Zn(II)

Cd(II)

Pb(II)

30.7

31.1

qL (mg,gL1)

Adsorbent

Smectites Montmorillonite [46] Acid-modified montmorillonite (AMM) [54]

21.1 2.2

4.0

2.8

76.9

0.6

1.6

40.9

24

0.6

23

24.1

63.7

181

4.2

27.7

Zeolites Natural Turkish clinoptilolite [55] Natural Sardinian clinoptilolite [56] Natural Ukrainian clinoptilolite [57]

13.03

25.8

Natural Turkish clinoptilolite [58]

13; 20.2

NaCl pretreated Turkish clinoptilolite [58]

80.6; 45.9

NaCl pretreated Mexican zeolite-rich tuff [59]

7.86; 9.18

Thiourea pretreated Mexican zeolite-rich tuff [59]

1.20; 11.60

Clays Natural Slovak bentonite clay [60]

27.7

Iron coated magnetic Slovak bentonite [60]

29.7

number of cycles resulting in a reduction of the amount of waste sorbent. Finally, the economic-environmental impact of synthetic sorbents is also favorable.

7.2.4.1

Inorganic synthetic sorbents

7.2.4.1.1 Silica gel Silica gel is composed of amorphous SiO2 in the form of hard irregular granules. The selectivity of commercially available sorbents depends on several factors, such as the type of modifier, its size, and specific hard-soft acid-base character. The common method of modifying silica gel is by reaction with an organosilane containing a chelating

204

Solid-Phase Extraction

functional group (e.g., 2,5-Dimercapto-1,3,4-thiadiazole; 2-Pyridinecarboxaldehyde phenylhydrazone; or others) [61]. For example, chemically modified silica gel containing aminothioamidoanthraquinone groups was used for sorption of Pb(II), Cu(II), Ni(II), Co(II) and Cd(II) at pH  3 with a selectivity order Pb(II) > Cu(II) > Cd(II); while for pH  4 Ni(II) > Co(II) with sorption capacities of 0.56, 0.30, 0.15, 0.12 and 0.067 mmol/g for Pb(II), Cu(II), Ni(II), Co(II) and Cd(II), respectively [61]. Silica gel impregnated with a mixture of Aliquat 336 and Eriochrome Blue SE was used for the separation of Cu(II), Cd(II), Pb(II), and Zn(II) with the relative capacity of the sorbent toward these metal increasing at higher pH [62]. A new type of modified silica sorbent was used for the selective extraction of Fe(III) from water using a pyridinium ionic liquid [63].

7.2.4.1.2 Metal oxides (e.g., aluminum oxides, magnesium oxides, ferric oxides, manganese oxides, titanium oxides, cerium oxides) Metal oxide sorbents at the nanometer scale, such as aluminum oxide, iron oxide, titanium oxide, manganese oxide, magnesium oxide, zirconium oxide, or cerium oxide have received increasing interest due to their sorption capability and high surface area [64]. Their sorption properties can be adjusted by varying the experimental conditions, such as sample properties (ionic strength, ion types, pH), experimental procedure (batch extraction or column flow), and temperature. An abbreviated comparison of the metal adsorption capacity for different nanosized metal oxides is given in Table 7.11 [64]. Table 7.11 Adsorption capacities of nanosized metal oxides (NMOs) for the extraction of heavy metals from aqueous samples [64]. Pb NMOs

Cu

Cd

Zn

Adsorption capacities of heavy metals [mg,g

Goethite (a-FeOOH)

149.25

Hematite (a-Fe2O3)

84.46

Hydrous ferric oxide (HFO)

20.27

Hydrous aluminum oxide (HAO)

20.27

Hydrous manganese oxide (HMO)

324.32

143.31

Maghemite (g-Fe2O3)

26.8

a-MnO2

82.6

TiO2

Cr

]

57.21

19.2

81.3

Ni L1

23.6

7.9e15.2

Al2O3

15.3

67.4 176.1

Modified Al2O3

100

16.3

ZnO

6.7

1600

CeO2

9.2

15.4e121.95

100

83.33

18.18

Metal-selective sorbents

205

7.2.4.1.3 Aluminosilicates Mesoporous silica-alumina oxide (97% SiO2, 3% Al2O) is a suitable sorbent for the extraction of heavy metals [65]. Toxic heavy metal ions, such as cadmium(II) and lead(II) can be extracted with a high adsorption capacity suitable for use at an industrial scale [66].

7.2.4.1.4 Cyanoferrates and hexacyanoferrates For the selective recovery of metal ions (also radionuclides), metal hexacyanoferrates are effective sorbents. Metal hexacyanoferrates are produced by the reaction of metal salts with potassium hexacyanoferrate [67,68]. A novel magnetic composite material based on magnetite and cobalt cyanoferrate nanoparticles (K2[CoFe(CN)]) was evaluated for the solid-phase extraction of radionuclide ions from aqueous solutions for the management of radioactive waste. The nanocomposite removes Cs(I), Eu(III) Co(II), Am(III) and Tc(VII) radionuclides from aqueous solutions with acceptable efficiency for waste management purposes [69].

7.2.4.1.5 Zeolites Synthetic zeolites are microporous crystalline aluminosilicates containing certain extra-framework cations. They are cation exchangers similar to natural zeolites but generally have a higher capacity [70]. They are more expensive than natural zeolites, but it is easier to tailor their physicochemical properties for specific applications [71]. The extraction of divalent metal ions (e.g., Cu2þ, Ni2þ, Cd2þ, and Pb2þ) from aqueous samples by two types of zeolites (synthetic Faujasite zeolite NaX) and zeolite A, LTA, was evaluated. The selectivity order for the NaX zeolite was Pb2þ > Cu2þ > Cd2þ>Ni2þ while for LTA Pb2þ > Cd2þ > Cu2þ > Ni2þ [70]. High sorption capacities were obtained, 715, 430, and 250 mg g1 for Pb(II), Cd(II), and Cu(II), respectively [71].

7.2.4.2

Organic synthetic sorbents

7.2.4.2.1 Ion-exchange resins Ion-exchange resins usually contain strong and weak cation and anion functional groups, which are bound to silica gel. They are classified based on the functionality of the ion-exchange sorbent. Classic ion-exchange sorbents are typically crosslinked styrene-divinylbenzene polymers with attached functional groups. Four types of ion-exchangers are distinguished: strongly acidic cation-exchange resins, weakly acidic cation-exchange resins, strongly basic anion-exchange resins, and weakly basic anion-exchange resins. Sorbents of the first group contain ion-exchange sites consisting of sulfonic acid groups. Weakly acidic cation-exchange resins usually contain carboxylic acid groups. Strongly basic anion-exchange resins contain quaternary ammonium functional groups. Weakly basic anion-exchange resins contain one or more primary, secondary and/or tertiary amine functional groups. The use of ion-exchange resins has been described numerous times, and here we present only a few representative results for the main groups of ion-exchangers. Dowex HCR S/H, a strong acid cation-exchange resin, was proposed for the separation of Cd(II) and Zn(II) [72], Dowex 50W synthetic resin - for Cu2þ,Zn2þ,Ni2þ,Cd2þ and Pb2þ [73],

206

Solid-Phase Extraction

Dowex 50WX4 nanocomposite - for Cr(VI), Ni(II), Cu(II), Cd(II), and Pb(II) [74], Dowex M-4195 - for Cu(II), Ni(II), Pb(II), Fe(III), Co(II), Mn(II) [75]) from water samples. The weak acid ion-exchanger Lewatit CNP 80 and a chelating derivative Lewatit TP 207, were evaluated for the extraction of Pb(II), Cu(II), Zn(II), Cd(II), and Ni(II) from aqueous solution [76]. The ion-exchange resin provided better performance than the chelating resin for the removal of the metal ions. It also showed higher selectivity for Ni and Cu than for Cd, Zn, and Pb [76]. The strong acid cation-exchange resin Lewatit S100 was suitable for the extraction of Cu(II), Zn(II), and Pb(II) ions [77]. Amberlite IR-120 cation-exchange resin, which has the sulfonate functional groups, appeared to be enough reactive and extremely pH sensitive. It was also proposed to be applied for adsorption and separation of Cu(II), Zn(II), Ni(II), Pb(II), and Cd(II) with an aim to their analytical determination in a wastewater sample. The sorbent shows better adsorption of Ni(II) and Zn(II) than Cu(II), Pb(II), and Cd(II) [78]. The Diphonix resin (containing diphosphonic, sulfonic, and carboxylic groups on a polystyrenedivinylbenzene matrix) was used to for the adsorption of Nd(III), Ce(III), La(III), Fe(III), Co(II), Ni(II), Cu(II), and Zn(II) metal ions in mixture from the regeneration of the Ni-MH batteries [79]. This resin had good adsorptive properties for rare earth elements and a lower capacity for other metals ions (i.e., almost 100% for the REEs and below 50% for the others). The selectivity series appeared to be La(III) > Fe(III) > Nd(III) > Ce(III) > Cu(II) > Zn(II) > Co(II) > Ni(II) [79]. The adsorption capacity was 59.41 (La), 26.69 (Ni), 0.65 (Nd), 1.00 (Ce), 6.65 (La), 3.48 (Fe), 1.74 (Co), 14.86 (Ni), 0.15 (Cu), 0.41 mg,g1 (Zn).

7.2.4.2.2 Chelating resins The most popular chelating sorbents contain iminodiacetic acid, polyamine, and glucamine groups bound to a highly porous styrene-divinylbenzene matrix or thioureabased resins. Different kinds of chelating sorbents have been designed for the selective extraction of heavy metals, alkali, alkaline earth, and transition metal ions. They bind selectively all metal ions which form complexes by coordination with specific chelation groups [80]. A number of methods have been used for their synthesis [80,81]. A selective chelating sorbent with iminodiacetate groups was evaluated for the extraction of heavy metal ions from water by batch experiments [82]. The sorption capacity followed the order: Pb(II) > Cd(II) > Zn(II) with a maximum adsorption capacity of 106 mg g1 Pb(II), 90 mg g1 for Cd(II), and only 15 mg g1 Zn(II) [82]. Examples of the use of chelating sorbents for the preconcentration and separation of trace metal ions, especially heavy metals, are summarized in Table 7.12 [83].

7.2.4.3

Hybrid sorbents

Synthetic hybrid inorganic-organic sorbents are a kind of artificial resins containing a mixture of inorganic and organic components. Hybrid sorbents typically possess favorable mechanical properties, enhanced chemical stability, and specific properties for the selective extraction of heavy metal ions [84]. A cellulose-nanoscale-manganese oxide composite (C-NMOC) was described for the extraction of Pb(II) ions from aqueous solution [85]. The adsorption capacity of CeNMOC (per gram of Mn present)

Metal-selective sorbents

207

Table 7.12 Chelating resins based on silica gel for the separation/or preconcentration of metal ions [83]. Functional group

Metal ions

Functional group

Metal ions

1,5-Bis(di-2-pyridyl) methylene thiocarbohydrazide

Hg, Zn, Cd, Co, Pb, Ni, Pt, Cr

Dendrimer-like polyamidoamine

Pd, Pt

1,8Dihydroxyanthraquinone

Fe, Co, Ni, Cu, Pb, Zn, Cd

Didecylaminoethylbetatridecylammonium

1-Aminoanthraquinone

Cu, Cr

Diethylenetriamine, mononaphthaldehyde, Monosalicyaldehyde

Fe, Ni, Cu, Zn, Cd, Pb

1-Nitroso-2-naphthol

Co

Dithioacetal derivatives

Hg

2-Amino-1-cyclopentene-1dithiocarboxylic acid

Ag, Hg, Pd, Cu, Ni, Cd Zn, Pt

Eriochrome black-T

Zn, Mg, Ca

2-Hydroxy-1naphthaldehyde

Cu, Zn, Cd, Hg, Pb

Formylsalicylic acid

Fe

2-Mercaptobenzothiazole

Bi, Pb, Cd, Hg, Au, Pt, Pd, Ag, Cu

Iminosalicyl group

Fe, Co, NiCo, Zn, Cd

3-Hydroxy-2-methyl-1,4naphthoquinone

Co, Cu, Zn, Fe

Methylthiosalicylate

Cd, Pb

3-Mercaptopropyl group

Hg, Au, Pd, Se,As

Morin

Zr, Be, Al, Sn

4-(2-Pyridylazo)resorcinol or 2-(2-Pyridylazo)-5dimethylaminophenol

Cd

N-propyl-Nc-[1-(2thiobenzothiazole)2,2c,2cctrichloroethyl]urea

Ag, Au, Pd

4-Amino-3-hydroxy-2-(2chlorobenzene)-azo-1naphthalene

Cr, Ni, Cu, Zn, Cd, Pb

N-tripropionate tetraazamacrocycles

U

4-Aminoantipyrene

Hg, Cd, Pb, Cu, Zn

o-Dihydroxybenzene

Cu, Pb, Fe, Zn, Co, Ni, Cd

Zn, Cu, Co, Pb

Continued

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Table 7.12 Chelating resins based on silica gel for the separation/or preconcentration of metal ions [83].dcont’d Functional group

Metal ions

Functional group

Metal ions

8-Hydroxyquinoline [quinolin-8-ol]

Cd, Pb, Zn, Cu, Fe, Mn, Ni, Co

o-Vanillin

Cu, Co, Fe, Zn

Acid red 88

Mg, Ni, Cu, Fe

Purpurogallin

Fe

Amidinothiourea

Ag, Au, Pd

Pyridinium ion

Co, Cu, Zn, Cd, Hg

Aminophenol, aminobenzoic acid

Fe, Co, Ni, Cu, Cd, Pb

Pyrocatechol violet

Cu, Ni, Mn, Co, Cu, Hg, Fe, Zn, Pd, Au, Pt, Ag

Aminopropyltriethoxysilane

Bi, Pb, Ni, Cu, Cd, Cr, V, Mn, Ag

Quaternary ammonium salts

Bi, Fe

Aurin tricarboxylic acid

Mo, Sn, Cu

Quercetin

Mo, Sn

Benzimidazole

Mn, Ni, Cd, Zn, Cu

Resacetophenone

Cu, Ni, Co, Zn, Fe

Calix-[4] arene tetrahydroxamate

Ni, Zn, Co, Pb, Cu, Mn

Thiosemicarbazide

Pd

Crown ether carboxylic acid

Se, Th, Rb, Na, K

Thiourea

Au, Ag, Pd

Cyanex 301

Bi

was several-fold greater than manganese oxide (b-MnO2) and even nanoscalemanganese oxide (NMO). Physisorption plays a dominant role in Pb(II) adsorption by both NMO and C-NMOC [85]. An organic-inorganic hybrid sorbent was prepared by self-hydrolysis, self-condensation, and cocondensation of the cross-linking agent (tetraethoxysilicate) and 3-(2- aminoethylamino)propyltrimethoxysilane monomer followed by gelation. This sorbent was used for the selective separation of Cd(II) from Zn(II) in aqueous solution. The selectivity coefficient for Cd(II)/Zn(II) was over 100 [86]. A mercapto-functionalized hybrid sorbent was used for sequestering Sb(III) species from aqueous samples [87]. A nanosized hybrid, biodegradable polymeric colloid was synthesized from nanosized MnO particles and polysaccharide [88]. The eco-friendly sorbent had a sorption capacity of 114 mg g1 for Ni(II).

Metal-selective sorbents

7.3

209

Conclusions

U The literature on sorption, in general, and on metal sorption, in particular, is rich and constantly growing. U So far, no theory has been formulated, which allows the prediction of the sorption properties of sorbents. U For the choice of a specific sorbent (both for metals and for other compounds), a review of existing sorption databases should be searched prior to starting experimental studies.

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Molecularly imprinted polymers Esther Turiel, Antonio Martín Esteban Departamento de Medio Ambiente y Agronomía, INIA, Madrid, Spain

8.1

8

Introduction

Nowadays, sophisticated and powerful analytical instrumentation is available, eventually allowing the determination of an organic compound in any kind of sample. Typically, chromatographic techniques coupled to common detectors (UV, fluorescence) or, more recently, mass spectrometry (MS) are routinely used in analytical laboratories for the determination of target compounds. However, direct injections of crude extracts are not possible even when using selective detection provided by MS, since matrix components can affect the ionization of analytes, hampering accurate quantification. Thus, a clean sample is generally recommended to improve separation and detection. Typically, several sample treatments are necessary for the unequivocal identification, confirmation, and quantification of target compounds. The main objectives of sample preparation are the removal of potential interferents, the preconcentration of target compounds (especially in environmental water samples), the derivatization (if needed) of analytes into a form more suitable for detection or separation, and finally, providing a robust and reproducible method independent of variations in the sample matrix. More recently, new objectives have emerged, including smaller sample sizes, higher selectivity, automation compatibility, and minimization of the amount of glassware, and organic solvents used [1]. Traditional liquideliquid extraction does not fulfill current requirements and has been displaced from laboratory practice by new extraction techniques, such as solid-phase extraction (SPE). Several sorbents with different properties are commercially available, and it is usually possible to find a description of an adequate SPE procedure for nearly all common target compounds. However, these sorbents are not highly selective and retain other components from the matrix along with the target compounds, which might hinder their quantification in the determinant step. Molecularly imprinted polymers (MIPs) are tailor-made synthetic materials with artificially generated recognition sites able to specifically rebind a target compound in preference to other closely related compounds. As shown in Fig. 8.1, these materials are obtained by polymerizing functional and crosslinking monomers around a template molecule, leading to a highly crosslinked three-dimensional network polymer. The monomers are chosen based on their capability to interact with the functional groups of the template molecule. Once polymerization is complete, the template molecule is extracted, leaving binding sites with shape, size, and functionalities within the polymer network complementary to the target compound. The resulting imprinted polymers are stable, robust, and resistant to a wide range of pH, solvents, and temperature.

Solid-Phase Extraction. https://doi.org/10.1016/B978-0-12-816906-3.00008-X Copyright © 2020 Elsevier Inc. All rights reserved.

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Figure 8.1 Preparation of MIPs.

Therefore, the behavior of MIPs emulates the interactions established by natural receptors to selectively retain a target molecule (i.e., antibodyeantigen) but without the associated stability limitations. In addition, the synthesis of MIPs is relatively simple and inexpensive, providing a clear alternative to the use of natural receptors. Three general approaches have been described for the synthesis of MIPs, namely, covalent, noncovalent, and semi-covalent approaches. The covalent approach [2] involves the formation of reversible covalent bonds between the template molecule and monomers before polymerization. Then, the template is removed from the polymer by cleavage of the corresponding covalent bonds, which are reformed upon rebinding of the target compound. This approach leads to a polymer with a rather homogenous population of binding sites, minimizing the existence of nonspecific sites, due to the high stability of template-monomer interactions. However, the covalent approach is rather restrictive due to the difficulty of designing an appropriate template-monomer complex in which covalent bond formation and cleavage are readily reversible under mild conditions. The semi-covalent approach offers an alternative intermediate approach [3,4]. In this case, the template is also covalently bound to a functional monomer, but the template rebinding is based only on noncovalent interactions. Finally, the noncovalent approach was introduced by Arshady and Mosbach [5] and is based on the formation of relatively weak noncovalent interactions (i.e., hydrogen bonds, ionic interactions, etc.) between the template molecule and selected monomers before polymerization. This approach is by far the most common for the preparation of MIPs owing to its simplicity and the availability of a wide variety of monomers. Regardless of how a MIP is prepared, it is evident that they are ideal materials for selective sample treatment prior to the determination of target compounds. Thus, their use in SPE, so-called molecularly imprinted solid-phase extraction (MISPE), is by far the most advanced technical application of MIPs. There is a large and expanding

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Figure 8.2 Number of publication on molecularly imprinted solid-phase extraction (MISPE) from 1996 to 2018 according to the Scopus database using the terms “molecularly imprinted solid phase extraction” or “MISPE.”

research activity in this area, evidenced by the number of papers published per year during the period 1996e2018, Fig. 8.2.

8.2

Preparation of molecularly imprinted polymers

8.2.1

General considerations

The proper selection of the template, monomer(s), crosslinker, and solvent (porogen) is key for the synthesis of a suitably selective MIP. Each variable has a strong influence on the overall performance of a MIPs in terms of affinity, selectivity, and loading capacity, etc. Their proper selection guarantees to a larger extent that MIPs with the appropriate properties are obtained for a particular application.

8.2.1.1

Template and monomer(s)

The first step in the preparation of MIPs consists of prearranging the template and the monomer(s) in a solvent. Monomer selection depends upon the characteristics of the target compound. The template molecule must contain functional groups capable of interacting with the monomer(s) to form a stable complex. For template molecules containing acidic groups, monomers with basic functional groups (e.g., 2- or 4vinylpyridine [VPy], diethylaminoethylmethacrylate [DEAEMA]) are preferred, whereas acidic monomers (e.g., methacrylic acid [MAA], trifluoromethylacrylic acid [TFM], itaconic acid [ITA]) are used to target bases. For carboxylic acids and amides, high selectivity was observed with primary amide-containing monomers

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(e.g., methacrylamide [MAAM]). Other neutral solvating monomers that commonly enhance the imprinting effect are N-vinylpyrrolidone (NVP) and hydroxyethylmethacrylate (HEMA). The structures for some common monomers are indicated in Fig. 8.3. It is important to note that since the template-monomer interactions are governed by an equilibrium process, a relatively large amount of monomer is used in order to displace the equilibrium to form the template-monomer complex. In general, a template-monomer molar ratio of 1:4 provides suitable stability to the templatemonomer complex, assuring the desired imprint effect. However, since the excess of free monomers leads to the formation of nonspecific binding sites, the loading, washing, and elution conditions for SPE must be correctly selected, as described later.

Figure 8.3 Structures of monomers commonly used in molecular imprinting.

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The solvent used during the prepolymerization step is also of prime importance since it has a direct influence on the strength of the template-monomer interaction. In general, solvents with a low dielectric constant, such as chloroform and toluene, are favored to stabilize hydrogen bonding and/or electrostatic interactions between monomer(s) and templates. Solvents with higher dielectric constants (e.g., acetonitrile) have also been used, but usually, the polymers obtained show a lower affinity to rebind the template. On the other hand, protic solvents, such as water and methanol, are not recommended, as they disrupt the template-monomer hydrogen-bonding interactions. Besides, apart from the above considerations, the template has to exhibit moderate to high solubility in the polymerization medium, As a consequence, in some cases, a compromise between the necessary solubility of the template and the strength of template-monomer interactions must be reached. Finally, the template size and shape has a strong influence on the selectivity of the MIPs, although it is not possible to establish a general rule for all systems. In general, slight structural differences near the functional group responsible for the interaction with the monomer lead to highly selective polymers preventing the binding of structurally related compounds [6,7]. However, in some cases, the absence or presence of groups far from the functional groups binding with the target compound has also allowed the preparation of highly selective imprinted polymers [8].

8.2.1.2

Crosslinker

The presence of a crosslinker is necessary to guarantee the stability of the templatemonomer complex during polymerization as well as to increase polymer porosity. It was shown that at least 50% of the total monomer in an MAA-ethylene glycol dimethacrylate (EDMA) system has to be EDMA; otherwise, no recognition can take place [9]. It is important to stress that the presence of a crosslinker not only preserves the binding sites but also has a direct influence on the physical and chemical properties of the polymer matrix. From this perspective, EDMA is the crosslinker most often used in methacrylate-based systems, since it provides mechanical and thermal stability, good wettability, and rapid mass transfer. Other crosslinkers, such as divinylbenzene (DVB) or trimethylolpropane trimethacrylate (TRIM), have also been used showing a similar performance to EDMA for certain applications. It is important to stress that the porogen (the solvent), together with the crosslinker, influences the polymer morphology in terms of the specific surface area and pore diameter. In general, a low surface area and low macroporosity are not desirable since they might lead to slow diffusion of the analyte to sites located in micro-pores, which reduces template recognition. In other words, a MIP with an inadequate morphology might prevent template recognition even when a solvent capable of stabilizing the template-monomer complex during the pre-polymerization step was employed.

8.2.2

Optimization of MIP formulations

As discussed above, several variables (kind and amount of monomer or nature of crosslinker and solvent) affect the final characteristics of a MIP in terms of capacity,

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affinity, and selectivity for the target compound. Thus, the identification of the optimum formulation for a particular MIP to be used for SPE might take several weeks of trial-and-error experiments. However, in most of the reports, such exhaustive optimization is rarely carried out, and certain standard formulations (e.g., 1:4:20 template:monomer:crosslinker molar ratio) are typically used [10]. For most applications, the MIPs present good affinity and selectivity for target compounds, but it is likely better performance would have been obtained if the MIP formulations were properly optimized. Therefore, some approaches dealing with the optimization of MIP formulations in a simple, fast, and rational way have been proposed to improve molecular recognition capabilities.

8.2.2.1

Computational approach

In this approach, uses molecular modeling software to design and screen a virtual library of monomers against the desired template. In this manner, it is possible to predict template-monomer interaction positions by calculating binding energies, facilitating the selection of the best functional monomer. Following this methodology, polymers with high binding capacity and selectivity have been obtained for different analytes [11e13]. In spite of the evident advantages of computational approaches in MIP design, its practice is far from routine. This situation might change in the future due to MIRATE (MIps RATional dEsign, http://mirate.di.univr.it/), a new open access online resource for the virtual design and refinement of molecularly imprinted polymer binding sites, described by Busato et al. [14], which does not require specific knowledge of molecular modeling techniques.

8.2.2.2

Combinatorial approach

The combinatorial approach [15,16] consists of the preparation of a large number of polymers in HPLC vials as small monoliths (mini-MIPs) as shown in Fig. 8.4. Then, the mini-MIPs are screened by determining the template release after incubation in the presence of a suitable solvent and by rebinding experiments. Due to the variety of parameters influencing the molecular recognition properties of MIPs, this approach can be a useful tool for facilitating the rapid screening and optimization of MIP formulations. This methodology was successfully employed for the selection of the best MIP formulation for different analytes, such as triazines [15,16], sulphonylurea herbicides [17], and bisphenol A [18], among others. It is important to note that the screening of mini-MIPs is restricted to equilibrium rebinding experiments, which differ from the nonequilibrium process that takes place in SPE. Besides, the fate of a mini-MIP (monolith) might differ from that of a polymer obtained by bulk polymerization after crushing and sieving. Thus, these limitations have to be considered, since it is not clear as to how well the results obtained from the screening of mini-MIPs can be extrapolated to typical SPE conditions.

8.2.2.3

Polymerization strategies

Bulk polymerization was the first strategy used to synthesize imprinted polymers, requiring grinding and sieving of the polymer mass to the desired particle size.

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CI Terbutylazine

O N H

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Figure 8.4 Scheme for the synthesis of mini-MIPs and subsequent screening of their recognition properties. Reprinted from Lanza F, Sellergren B. Method for synthesis and screening of large groups of molecularly imprinted polymers. Anal Chem 1999;71:2092e6, Copyright (1999) with permission from the American Chemical Society.

This process is tedious and time-consuming, and the particles obtained possessed a heterogeneous particle size distribution with poor binding site accessibility for the target compound. Although these polymers can be useful for most SPE applications, their drawbacks prevent their large-scale production. Accordingly, new polymerization methodologies to obtain MIP beads with proper physical characteristics (size, porosity, pore volume, surface area) have been developed over the last few years. Among these new techniques, multistep swelling and polymerization [19], precipitation polymerization [20] and polymerization into the pores of silica beads, with the latter etched away with NH4HF2 after polymerization [21], have been used for the preparation of MIPs for SPE. Of these, precipitation polymerization is the most straightforward strategy for the rapid preparation of MIP in high yield. Simply, this method consists of carrying out the polymerization of monomers in a large amount of a suitable solvent (w4%e6% w/v). However, it is important to stress that the compatibility between the crosslinker and the porogen (solvent) is of vital importance for the success of this methodology; otherwise, agglomerates instead of independent beads might be obtained. In any case, the simplicity of the experimental procedure and the elimination of crushing and sieving steps, which eases large-scale production, justify the use of this methodology for the preparation of MIPs for SPE.

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8.2.2.4

Template bleeding

Once the MIP has been prepared, regardless of the used polymerization strategy, the template must be removed in order to obtain free binding sites. Template removal is usually carried out by washing the polymer repeatedly with a solvent capable of disrupting the template-monomer interactions or by Soxhlet extraction. However, it is known that even after exhaustive washing by different approaches (thermal annealing, microwave assisted extraction, Soxhlet extraction, and supercritical fluid template desorption) [22], traces of template remain in the polymer network. Consequently, some template leakage might be detected during the elution step in MISPE protocols, although in most applications, no template bleeding was observed at the concentration levels of the study. Thus, although template bleeding exits, it is not necessarily a major problem for typical applications. Nevertheless, minimizing template bleeding by using an analog of the target compound as a template is recommended. In this manner, the bleeding of the template does not interfere in the quantification of the target compound. The use of a closely related compound to target compound as template was originally proposed by Andersson et al. [23] for the extraction of sameridine from human plasma. Later, this approach was followed by others allowing the quantification of different analytes in a precise and accurate manner. In spite of such success, template-analog imprinted polymers possess an inferior molecular recognition capability compared to MIPs prepared using the analyte as a template. The best way to improve the molecular recognition capability is to use a template analog whose shape and functionality are as similar to those of the target compound as possible. An interesting alternative is the use of a stable isotope labeled compound as the template molecule, so-called isotope imprinting. This approach was originally proposed by Sambe et al. [24] for the preparation of a restricted access material-MIP (RAM-MIP) for bisphenol A-d16 (BPAd16). The molecular recognition ability of this polymer for BPA, BPA-d6, BPA-d16, and related analogs was evaluated for the direct injection of BPA in serum by column-switching LCeMS. In this case, although template bleeding did occur, BPA was detected by selective ion monitoring. Obviously, although the potential of isotope imprinting to overcome the template bleeding problem is evident, its applicability is limited by the availability of the corresponding isotopic labeled compound analog and the use of MS for detection.

8.3 8.3.1

Molecularly imprinted solid-phase extraction (MISPE) Off-line protocols

Typically, as in other SPE procedures, a small amount (15e500 mg) of imprinted polymer particles is packed into conventional polyethylene cartridges. Then, the MIP cartridge is conditioned with an appropriated solvent, and the sample is loaded. After a washing step for removal of nonspecifically bound matrix components, the

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analytes are eluted, ideally free of coextractives. The sample is usually loaded onto the MIP cartridge in a low-polarity solvent (e.g., acetonitrile, dichloromethane, toluene), which in most cases, is the same solvent as the porogen for the MIP synthesis to maximize media specific interactions. The target compounds are eluted with a solvent capable of disrupting the typical noncovalent interactions between the target compounds and the imprinted polymer. Nevertheless, aqueous samples can also be loaded directly onto MIP cartridges [25]. In this case, MIPs behave like a reversed-phase sorbent controlled by nonspecific interactions between the target compounds and matrix components and the polymer matrix. Then, during the washing step, matrix components are removed using an appropriate solvent capable of redistributing nonspecifically bound analytes to the selective binding sites. Unfortunately, the success of this procedure is not always achieved promoting interest in the synthesis of water-compatible MIPs with recognition abilities in aqueous media. This is achieved by incorporating hydrophilic surface properties to the polymer to reduce nonspecific hydrophobic interactions. Water-compatible MIPs have been prepared using polar porogens [26], hydrophilic comonomers (e.g., 2-hydroxyethyl methacrylate, acrylamide) or cross-linkers (e.g., pentaerythritoltriacrylate, methylenebis(acrylamide)) [27e29] and/or specially designed monomers capable of stoichiometric interactions with the template functional groups [30,31]. The proposed water-compatible MIPs provided recognition of target compounds to a certain extent in aqueous media and further development in this field can be expected in coming years. As mentioned above, the inclusion of a washing step is necessary to maximize the specific interactions between the target compound and the imprinted polymer with the simultaneous elution of nonspecifically bound interfering compounds. The presence of nonspecific binding sites is attributed to the high amount of monomer used during MIP synthesis by the noncovalent approach. Consequently, the excess of free monomers, randomly incorporated in the polymer matrix leads to the formation of nonselective binding sites. These nonspecific binding sites can lead to the coextraction of some matrix components which can interfere in the chromatographic determination of target compounds after MISPE, even after an exhaustive washing step. Besides, the loss of MIP performance due to matrix compounds strongly bound to the polymeric matrix has been observed [32,33]. These drawbacks can be circumvented by introducing a previous clean-up step using a nonimprinted polymer (NIP), a polymer prepared in the same conditions as the corresponding MIP but in the absence of the template molecule, so-called two-step MISPE, applied successfully to the extraction of triazines and phenylurea herbicides from vegetable extracts [33,34]. Briefly, in this approach, the sample is loaded onto the NIP with nonspecific retention of both target compounds and matrix components. Then, during the washing steps, the target compounds (and a small amount of interfering compounds) are eluted, while the majority of matrix compounds remain bound to the NIP. Subsequently, the washing solution is loaded onto the corresponding MIP for further cleanup. In addition, by this approach the MIP is protected from high loading of matrix compounds, thus increasing its durability and preservation of its recognition properties.

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In spite of the success of the two-step MISPE approach, the best way of reducing nonspecific interactions is by using imprinted polymers synthesized by the covalent or semi-covalent approaches. This statement was nicely demonstrated by Cacho et al. [35] for the MISPE of triazines from soil and vegetable extracts using a semi-covalent imprinted polymer prepared with propazine methacrylate as a template molecule. Compared to a noncovalent MIP using propazine as a template, a markedly lower nonspecific binding of matrix components was observed leading to lower detection limits, much lower than those obtained using a noncovalent MIP.

8.3.2

Online protocols

The first online MISPE procedure coupled to HPLC was proposed by Masqué et al. [36] for the selective extraction of 4-nitrophenol from a mixture of phenolic compounds in river water samples. In general, in this format, a small precolumn packed with the imprinted polymer (w25e50 mg) is placed in the loop of a six-port injection valve. After loading the sample and washing out interfering compounds, the target compounds are eluted by the mobile phase and then separated by liquid chromatography. However, in most cases, there is poor compatibility of the mobile-phase required for the chromatographic separation and the elution solvent required to desorb the target compounds from the MIP pre-column. Consequently, the number of reports utilizing online MISPE is scarce. Alternatively, different online MISPE-HPLC systems, as illustrated in Fig. 8.5, can be employed to direct the elution solvent from the MIP precolumn to the injection loop (Fig. 8.5A) and subsequently injected into the chromatographic system [37] or by mixing of the elution solvent with an aqueous-rich solvent (Fig. 8.5B) before reaching the separation column [38]. Both approaches increase the complexity of the online system requiring the incorporation of additional instrument components (e.g., pumps). Besides, in online systems, MIPs must provide adequate permeability and fast mass transfer of analytes, requiring the synthesis of MIPs with improved performance characteristics. In this regard, molecularly imprinted polymer filaments (MIPFs) as the extraction phase have been proposed, providing improved permeability compared with monolithic or particle-packed columns with finely dispersed sorbents [39]. Besides, the small internal diameter tubing used could produce high linear flow rates at the surface of the coatings, resulting in the fast mass transfer of target compounds between the solid and liquid phases.

8.3.3

In-line protocols

The direct coupling of a MIP column in-line with the detection system might allow the extraction, enrichment, separation, and determination of target compounds in a single step owing to the high selectivity of the MIPs. The very first in-line MISPE protocol coupled with UV-detection was described by Sellergren for the determination of pentamidine in urine samples [40]. Dilute urine, 100 mL, was loaded on a column packed with MIP particles, washed with 100 mL of a buffer solution (pH 9), and the target compound pentamidine eluted in w1.5 mL of buffer at pH 3 with UV

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(A)

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Figure 8.5 Alternative online MISPE-HPLC set-ups. Reprinted from Theodoridis G, Zacharis C, Tzanavaras P, Themelis D, Economou A. Automated sample preparation based on the sequential injection principle: solid-phase extraction on a molecularly imprinted polymer coupled on-line to high-performance liquid chromatography. J Chromatogr A 2004;1030:69e76, Copyright (2004), and Ou J, Hu L, Hu L, Li X, Zou H. Determination of phenolic compounds in river water with on-line coupling bisphenol A imprinted monolithic precolumn with high performance liquid chromatography. Talanta 2006;69:1001e6, Copyright (2006) with permission from Elsevier.

detection at 270 nm. However, a large volume of washing solvent was necessary to remove a high amount of matrix compounds retained by the MIP column by hydrophobic interactions, requiring a long analysis time. Later, since the recognition capabilities of MIPs are enhanced in the organic media, Turiel et al. developed a method for the determination of the fungicide thiabendazole (TBZ) in acetonitrile-based extracts of fruit samples [41]. In this case, the extracts (50 mL) were directly injected onto a column packed with TBZ-imprinted polymer beads using methanol as a mobile-phase. After 2.4 min, the mobile-phase was

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switched to a methanol-acetic acid mixture (80:20, v/v) in 0.1 min, maintaining these conditions constant for 5 min before returning to the initial conditions. Due to the high selectivity of the MIP column, the target compound was retained on the column, while interferences were rapidly eluted, facilitating the quantification of TBZ in less than 15 min. Peak broadening and peak tailing phenomena are typically observed in the elution profiles of in-line MISPE procedures due to slow mass transfer and the existence of a heterogeneous binding-site distribution, even for imprinted beads. As an alternative, surface imprinting, the formation of a thin film of imprinted polymer grafted onto the surfaces of preformed beads, may solve some of these drawbacks. MIP microspheres with core-shell morphology and narrow particle-size distributions were proposed for in-line MISPE of TBZ from citrus fruits and orange juice samples [42,43]. By using core-shell particles, a clear improvement in the chromatographic performance was achieved, demonstrating that the slow mass-transfer kinetics associated with conventional MIP stationary phases can be dramatically improved by surface-imprinting techniques. As an example, Fig. 8.6 shows the chromatogram obtained after the direct injection of a nonspiked orange juice sample onto a column packed with core-shell TBZ-imprinted microspheres. The core-shell MIP column permitted retention of the target analyte while the interferences were eluted rapidly.

Figure 8.6 Chromatogram for the direct injection of a nonspiked orange juice sample onto a column packed with core-shell TBZ-imprinted microspheres. Chromatographic conditions were as follows: mobile phase consisting of acetonitrile, 2 min; mobile phase consisting of acetonitrile-methanol (95:5), 8 min; mobile phase consisting of methanol-acetic acid (50:50), 5 min; linear gradient to initial conditions, 5 min; conditioning under initial conditions, 5 min. Reprinted from Barahona F, Turiel E, Cormack PAG, Martin-Esteban A. Synthesis of core-shell molecularly imprinted polymer microspheres by precipitation polymerization for the in-line molecularly imprinted solid-phase extraction of thiabendazole from citrus fruits and orange juice samples. J Sep Sci 2011;34:217e24, Copyright (2011) with permission from John Wiley & Sons.

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In the above in-line MISPE procedures, analyte elution is performed by a step solvent-switch. Alternatively, pulsed elution (MISPE-PE), in which a small volume of elution solvent is used, was proposed for the determination of theophylline in chloroform diluted serum samples [44]. In this first work, samples were injected onto a theophylline-imprinted polymer packed into a stainless steel column using chloroform as a mobile phase. After interfering compounds were washed out from the column, theophylline was eluted free of coextracted interferents by injection of 20 mL of methanol and determined spectrophotometrically at 270 nm. Later, MISPE-PE evolved to MISPE with differential pulsed elution (MISPE-DPE), in which successive 20 mL pulses of different solvents are applied to wash out interfering compounds and the analyte fraction [45]. It is important to note that analyte enrichment through injection of large volumes of sample is possible, providing a high sensitivity due to the narrow band obtained by pulsed elution.

8.3.4

Improved batch protocols

Batch MISPE was first used for the extraction of sameridine, where the MIP particles were incubated for 1 h with spiked human plasma. After the washing step, sameridine was eluted for quantification by gas chromatography [23]. However, such protocols require additional centrifugation and/or filtration steps and have largely been displaced by the conventional off-line SPE methods. However, batch MISPE is experiencing a rebirth, with the use of MIP-modified magnetic nanoparticles, since after extraction, such particles can be separated from the matrix by a magnet, thereby avoiding tedious filtration and/or centrifugation steps. Several procedures have been described for the synthesis of magnetic core-shell MIP particles. In general, magnetite nanoparticles are initially encapsulated in silica through a typical sol-gel reaction using tetraethyl orthosilicate (TEOS) forming hybrid Fe3O4@SiO2 particles. Polymerizable double bonds, necessaries for the subsequent surface imprinting, are introduced on particle surface by the reaction of Fe3O4@SiO2 particles with suitable reagents. Finally, the MIPs are formed on the surface of Fe3O4@SiO2 by the copolymerization of vinyl end groups with the typical polymerization mixtures used in molecular imprinting. Once the magnetic MIP beads are obtained, they can be used in batch MISPE procedures. This approach was used for the extraction of various analytes in different samples, such as tetracycline antibiotics from egg and tissue [46], microcystins in environmental waters [47], and triazines in soils [48], among others. For further details, ref. [49] is recommended. Micro solid-phase extraction (m-SPE) can be considered a variant of batch SPE, which consists of retaining a sorbent material in a poly(propylene) (PP) flat-sheet membrane envelope. Target analytes diffuse freely through the membrane pores and are retained by the sorbent material while other matrix compounds are hindered by the membrane. For improving selectivity for the extraction of phenolic compounds from environmental water samples, MIP particles were employed as the sorbent material in the so-called molecularly imprinted polymer-based m-SPE (MIP-m-SPE) [50] and for the extraction of ochratoxin A from coffee, grape juice, and urine [51], and cocaine and its metabolites from human urine [52].

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Alternatively, the use of a poly(propylene) capillary instead of a flat-sheet membrane envelope has been proposed to reduce the amount of MIP particles and the consumption of organic solvents [53,54]. Although few papers have been published so far, the growing interest in MI-m-SPE suggests that this should change in the near future driven by the simplicity of operations.

8.4

Selected applications

Most of the reports highlight the great selectivity provided by MIPs, but few properly report the advantages of MISPE compared to the traditional methods using nonselective sorbents. A notable exception is a work by Andersson et al. [55] comparing MISPE with mixed-mode SPE for the analysis of benzodiazepines in postmortem hair samples. First, hair samples were washed and then incubated in methanol25% ammonium hydroxide solution (20:1) overnight. The extract was then reconstituted in a buffer for SPE or toluene for MISPE. After extraction and cleanup, sample extracts were reconstituted in the mobile phase and analyzed by LC-MS/MS. The MISPE method provided higher recovery with lower limits detection limits for diazepam while the SPE method failed to detect it. The authors conclude that not only did MISPE provide superior extraction performance but was also a simpler and more time-efficient procedure. Another significant contribution to the analysis of environmental wastewater samples is the analysis of b-blockers by liquid chromatographyequadrupole-linear ion trap mass spectrometry [56]. In this work, the performance of a commercially available b-blockers-receptor MIP was compared with a generic mixed-mode SPE phase (Oasis HLB). Recoveries of b-blockers by both sorbents were similar, but the MISPE method required a significantly smaller sample volume (25 mL) compared with conventional SPE (100e200 mL). As a consequence, a reduced matrix effect on the separation and detection was observed. It was found that the extracts from MIP resulted in minimal signal suppression, whereas those from Oasis HLB showed significant suppression of the analyte signal. In general, the effective clean-up achieved by MISPE are associated with considerable savings in time and costs. This was clearly demonstrated by Mohamed et al. [57] for the determination of chloramphenicol in milk (raw milk, skimmed milk, and milk powder). The advantages of a commercial MIP were evaluated by comparing its performance with that from a standard generic SPE method. The traditional approach involved several steps, including protein precipitation, SPE cleanup, and three liquid-liquid extraction steps. However, the MIP-based method only required a prior centrifugation step. Obviously, it allowed a significant increase in sample throughput (18 samples processed within 3 h compared with 8 h with the classical approach). In addition, the generic SPE method required LC-electrospray ionization (ESI)-MS/MS to achieve the required sensitivity and selectivity, whereas the MISPE protocol allowed the use of LC-UV or fluorescence detectors to achieve the same performance.

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A further example of the superior performance of MIPs in food analysis was demonstrated for the extraction of fluoroquinolones from baby food [58]. In this work, the target compounds were isolated by ultrasound-assisted extraction, and the extracts loaded onto a ciprofloxacin-imprinted polymer or a strong anionexchange resin. After, the corresponding washing steps, the target compounds were eluted and for analysis by HPLC-UV. An example of typical chromatograms for the analysis of spiked and unspiked baby food samples is shown in Fig. 8.7 for conventional SPE using strong anion-exchange cartridges (Fig. 8.7A and B) and by MISPE (Fig. 8.7C and D). A clean baseline was obtained using MISPE allowing an-order-of magnitude reduction of detection limits compared with conventional SPE using strong anion-exchange cartridges.

Figure 8.7 Chromatograms obtained after ultrasound-assisted extraction of baby-food samples and clean-up using strong anion exchange (A and B) and MIP cartridges (C and D). (A and C) Sample spiked at 1 mg g
1; (B and D) unspiked sample. Peak assignment: 1 ¼ Enoxacin; 2 ¼ Norfloxacin; 3 ¼ Ciprofloxacin; 4 ¼ Danofloxacin; 5 ¼ Enrofloxacin. Reproduced from Díaz-Alvarez M, Turiel E, Martín-Esteban A. Selective sample preparation for the analysis of (fluoro) quinolones in baby food: molecularly imprinted polymers versus anionexchange resins. Anal Bioanal Chem 2009;393:899e905, Copyright (2009) with permission from Springer.

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Conclusions

MIPs are excellent sorbents for customized sample preparation by SPE. MIPs are capable of cleaning up complex samples facilitating the selective isolation of target compounds in a rapid and robust manner. In general, the selectivity provided by MIPs allows lower detection limits to be obtained with conventional detectors (spectrophotometers, spectrofluorimeters, etc.) Although MISPE can be considered a robust technique, some improvements remain necessary, especially those related to the selection of polymerization mixtures, industrial scale-up through alternative polymerization strategies, and the development of water-compatible sorbents, among others. The development of new SPE formats with improved mass-transfer characteristics and the coupling of MIPs to other extraction techniques are expected in the near future. The development of micro-MISPE devices is likely to be one of the main areas of activity in the coming years.

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Magnetic nanoparticle sorbents Bin Hu, Man He, Beibei Chen Department of Chemistry, Wuhan University, Wuhan, Hubei, China

9.1

9

Introduction

For magnetic solid-phase extraction (MSPE), the magnetic sorbents employed have a great influence on the separation and enrichment speed, enrichment factor, selectivity, antiinterference ability, and reproducibility of the MSPE-based methodologies. An ideal magnetic sorbent should have the following advantages: (i) strong magnetism to achieve fast magnetic separation; (ii) good dispersion, so as to improve the adsorption/desorption kinetics; (iii) large specific surface area, suitable porosity, and easy to modify, to provide abundant adsorption sites, which help to improve the adsorption capacity and extraction efficiency/recovery of target compounds; (iv) good selectivity/antiinterference capability, which helps to improve the ability of the method to tolerate complex matrixes; (v) good stability, can withstand acid and alkali environments, ultrasound, stirring, and oscillation treatment; (vi) reusable, namely, reversible adsorption; (vii) mild adsorption and desorption conditions; (viii) easy preparation and good reproducibility of preparation; (ix) low cost, and easy to obtain raw materials and low sorbents consumption during extraction; (x) environmentally friendly with low reagent consumption in its preparation and in the extraction process. Magnetic nanoparticle (MNP) sorbents used for MSPE are nanometer-sized structures with good magnetism and available with various functional groups. MNP sorbents with superparamagnetism can be easily and rapidly separated from aqueous solution with the aid of an external magnetic field, facilitating the isolation of the target compounds adsorbed on the MNPs from the sample matrix. MNP sorbents can be functionalized with specific groups, e.g., hydroxyl, carboxyl, sulfonic acid, amino, mercapto, or chelating functional groups, resulting in favorable selectivity for various target compounds. So far, a variety of materials have been introduced, including silicabased, carbon-based, metal/metal oxides, metal-organic frameworks, porous organic polymers, mesoporous materials, and imprinted and restricted access materials.

9.2 9.2.1

Preparation techniques Synthesis of magnetic nanoparticles

MNP sorbents with core-shell structures consisting of a magnetic core and a coating or magnetic composites prepared from magnetic particles and other materials are the most common types. There are many types of MNPs that can act as a magnetic core, including nickel (Ni), iron (Fe), cobalt (Co), and their alloys/oxides. Of these, iron Solid-Phase Extraction. https://doi.org/10.1016/B978-0-12-816906-3.00009-1 Copyright © 2020 Elsevier Inc. All rights reserved.

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Solid-Phase Extraction

Table 9.1 Saturation magnetization of some magnetic metal oxides [1]. Oxides

Magnetization saturation (emu gL1)

g-Fe2O3

74

Fe3O4

84

Fe2O3-Fe3O4

80

CoO$Fe2O3

65

oxide (g-Fe2O3, Fe3O4) MNPs are the most widely used [1]. Table 9.1 lists the saturation magnetization of common metal oxides used in MNP sorbents; a saturation magnetization of above 20 emu g
1 is beneficial for magnetic separations. Normal synthesis methods for MNPs, include coprecipitation, hydrothermal/ solvent thermal method, microemulsion method, sol-gel method, oxidation method, hydrolysis method, thermal decomposition method, and ultrasonic precipitation method. Coprecipitation is currently the most widely used method [2]. (i) Coprecipitation method: During the precipitation reaction, the metal precursor (Fe2þ, Fe3þ) is dissolved in an aqueous solution, followed by the addition of precipitating reagent (e.g., alkali solution) to form insoluble solids. MNPs prepared by this strategy, usually have a wide size distribution and irregular morphology. This method can produce various magnetic materials, including spinel ferrite, perovskites, metals, and alloys. Based on the simple coprecipitation method, Lin et al. [3] prepared Fe3O4 nanoparticles (NPs) in a rotating packed bed, and relevant factors affecting the particle size of the NPs were investigated. It was shown that Fe3O4 NPs with smaller particle size could be obtained when higher rotational speeds and higher reactant and precipitant flow rates were employed. However, the concentration of the reactants and precipitant hardly affect the particle size of Fe3O4 NPs. The prepared Fe3O4 NPs had an average particle size of about 5.1 nm and a saturation magnetization of 50 emu g
1. The main advantage of the coprecipitation method is that large quantities of products can be produced by a single synthesis. The simple operation and short reaction time make it suitable for industrial production. However, the particle size is difficult to regulate, and the NPs are prone to agglomeration producing a wide distribution of particle sizes. (ii) Hydrothermal method: It is a solution reaction carried out in a reaction vessel or high-pressure reactor, usually at temperatures of about 200 C and pressures greater than 2000 psi. In a supercritical state, water works as a reactant and increases the kinetics of the hydrolysis reaction. At high temperatures, the solubility of most metal ions increases, and the viscosity of water decreases, with the metal ions exhibiting increased fluidity. This allows Ostwald ripening to proceed at a faster rate improving the uniformity of the precipitation. In the hydrothermal reaction, the size and morphology of the product can be adjusted by controlling the reaction time and temperature. Choice of precursor and the reaction condition such as pH significantly affect the phase purity of the NPs.

Magnetic nanoparticle sorbents

237

Ding et al. [4] synthesized Fe3O4 nanorods by an ethylenediamine-assisted hydrothermal reaction. The Fe3O4 nanorods have a cubic spinel crystal form, with a particle diameter of about 5 nm and length of about 5 mm. The ethylenediamine acts as a mineralizer and template in the synthesis of the Fe3O4 nanorods. Its concentration plays a decisive role in controlling the morphology and composition of the nanorods. Li et al. [5] synthesized Mn-Zn ferrite nanoparticles using the hydrothermal method. It was found that a low Fe content and the hydrothermal temperature was important for the synthesis of single-phase Mn-Zn ferrite nanoparticles. The hydrothermal temperature significantly affected the morphology and magnetic properties of the product. Mn-Zn ferrite nanoparticles synthesized at less than 180 C exhibited a saturation magnetic strength of 45.5 emu g
1. The NPs prepared by the hydrothermal method have high purity and controllable crystal forms. Since the reactor needs to be able to withstand high temperatures and pressures, the hydrothermal reaction requires suitable equipment for the reaction, which increases the cost. Furthermore, the reaction is carried out in a closed reactor, which makes it difficult to observe the growth of crystals. (iii) Microemulsion method: In this method, a solution containing surfactants spontaneously form thermally stable and isotropic globular aggregates at the oil/water interface. Microemulsions can be divided into two types, oil-in-water (O/W) and water-in-oil (W/O). By changing the ratio of the aqueous phase, the oil phase, and the surfactant, the morphology of the product can be effectively regulated. Inouye et al. [6] used the microemulsion method to prepare Fe3O4 and g-Fe2O3 nanoparticles with a narrow size range from ferrous oxide salts. The reaction was carried out in an emulsion containing sodium dioctyl sulfosuccinate and isooctane. Kelly et al. [7] prepared Mn-Zn ferrite NPs with a spinel structure and narrow particle size distribution in an O/W microemulsion. By changing the composition of the O/W microemulsion and the type/concentration of the precipitant, the particle size, crystallinity, and magnetic properties of the products can be effectively regulated. Compared with other methods, the microemulsion method produces magnetic nanoparticles with narrow and controllable particle size, resulting in better uniformity. Since the particles are encapsulated by the surfactant, the resultant MNPs are less prone to agglomeration and have better monodispersity. Besides, the use of various surfactants would endow the magnetic NPs with different surface properties. On the other hand, the use of surfactants makes it essential to incorporate a cleaning step in the synthesis to remove surfactants from the surface of the magnetic NPs, which increases the cost. (iv) Sol-gel method: A metal alkoxide is dissolved in a solvent to form a sol, which is hydrolyzed and condensed to form a gel, and then subjected to heat treatment, annealing, and/or sintering, resulting in NPs. In the sol-gel process, factors such as solvent type, temperature, choice of precursor, catalyst, pH, and additives, need to be considered. They affect the kinetics of the hydrolysis and condensation reactions and the growth of the magnetic NPs. Besides the hydrolysis and condensation reactions, pH affects the isoelectric point and stability of the sol, which in turn affects particle aggregation and particle size. By changing the factors affecting the rate of the hydrolysis and condensation reaction, the structure and properties of the gel can be effectively regulated.

238

Solid-Phase Extraction

Cui et al. [8] synthesized nearly monodisperse a-Fe2O3, g-Fe2O3, and Fe3O4 nanoparticles through a low-temperature sol-gel route. Two steps were involved in the preparation process. The first step was the reaction between FeCl2 and propylene oxide in boiling ethanol to form a sol-gel followed by a drying step. The structure of iron oxide can be adjusted by changing the drying condition for the sol. The reaction between propylene oxide and [Fe(H2O)6]2þ gradually promotes the hydrolysis and condensation of metal ions, resulting in uniform nucleation of iron oxide in the solution. In addition, this method has a high yield with about 60 g of nanoparticles obtained in a single synthesis. Zhang et al. [9] synthesized Fe3O4 aerogels by the sol-gel method using a supercritical carbon dioxide drying technique. FeCl3$6H2O, FeCl2$4H2O, and propylene oxide were used as a precursor and gelling accelerator, respectively. The as-prepared aerogels had a three-dimensional network structure consisting of interconnected NPs. The Fe3O4 aerogels possessed a low density (0.25e0.39 g m
3), large specific surface area (60e330 m2 g
1), and superior saturation magnetization (15e52 emu g
1). The structure and magnetic properties of the aerogels can be controlled by varying the solution concentration, molar ratio of propylene oxide/Fe3þ, and the calcination temperature. MNPs prepared by the sol-gel method have high crystallinity and controllable particle size with good uniformity. On the other hand, the preparation process is tedious, and the reaction period is relatively long. (v) Oxidation method: Adding stabilizer (water-soluble polymer, e.g., polyethylene glycol, polyvinyl alcohol) and oxidant to the aqueous solution containing metal ions can produce metal oxide. For example, a magnetic fluid can be obtained by adding an alkaline solution to a solution containing Fe2þ while simultaneously adding H2O2 or introducing air. MNPs prepared by this method are close to a spherical shape with a large particle size distribution. The average particle diameter of MNPs obtained by this method is often larger than that prepared by the coprecipitation method. By controlling conditions, such as air flow rate and reaction temperature, MNPs with different particle sizes can be obtained. (vi) Hydrolysis method: This approach can be divided into two main routes, titration hydrolysis, and a Massart hydrolysis [10]. In the former, a mixed solution of ferric and ferrous salts with a molar ratio of n(Fe3þ): n(Fe2þ) of 2:1 is prepared by the dropwise addition of dilute alkali solution gradually increasing the pH of the mixed solution, hydrolyzing the iron ions to form Fe3O4 NPs. In the latter case, the mixed solution containing ferric and ferrous salts (n(Fe3þ):n(Fe2þ) of 2:1) is directly added into an alkaline aqueous solution, and the iron ions are immediately hydrolyzed to form Fe3O4 NPs. The reaction conditions are mild and do not require special equipment. on the other hand, the reaction temperature, reactant concentration, precipitant concentration, precipitant addition rate, agitation, and pH affect the particle size distribution of the products. (vii) Thermal decomposition method: In this method, the organic metal precursor is decomposed at high temperature to prepare NPs. It is one of the simplest methods for NPs preparation. Compared with the other methods, the main advantage is that the decomposition temperature is low and controls the growth of the nanoparticles. Typically, polymers, organic blocking agents, or structural entities are used to limiting the size of the resultant NPs.

Magnetic nanoparticle sorbents

239

Jana et al. [11] prepared nearly monodisperse Fe3O4 NPs by the pyrolysis of iron fatty acid salts. Regulation of the size and shape of the NPs can be achieved by changing the reactivity and concentration of the precursors. The reactivity can be tuned by changing the chain length and concentration of fatty acid. Asuha et al. [12] synthesized Fe3O4 powder by the thermal decomposition of iron-urea complex ([Fe(CON2H4)6](NO3)3) in a closed vessel. The method is simple with good reproducibility. One of the decomposition products of [Fe(CON2H4)6](NO3)3 is CON2H4, which is a reducing agent, and plays an important role in the formation of Fe3O4. The particle size of Fe3O4 NPs can be adjusted by changing the reaction temperature. Increasing the reaction temperature from 200 to 300 C increases the average particle size of the nanoparticles from 37 to 50 nm. As the saturation magnetic strength of Fe3O4 NPs is closely related to the particle size, the saturation magnetic strength also increases from 71 to 89 emu g
1. MNPs prepared by thermal decomposition method have the advantages of good dispersibility, adjustable particle size, and narrow size distribution, while, the use of organic reagents during the preparation process may increase environmental pollution. (viii) Ultrasonic precipitation method: Compared to conventional mixing techniques, ultrasonication makes it easier to obtain a homogeneous dispersion. The “ultrasonic vaporization bubbles” [13] produced by ultrasonic waves can form a local high temperature and pressure environment to accelerate chemical reactions. Ultrasonic precipitation avoids the uneven concentration dispersion of liquids, reduces the occurrence of agglomeration, and greatly increase the reaction speed, favoring the formation of small particles. Vijayakumar et al. [14] prepared Fe3O4 NPs with a particle size of about 10 nm from an aqueous solution of iron acetate under high-intensity ultrasonic radiation in an argon atmosphere.

9.2.2

Surface modification of magnetic nanoparticles

In theory, there are four types of interparticle forces in magnetic nanoparticles: van der Waals forces, magnetic forces, electrostatic repulsion forces, and steric hindrance. The first two forces tend to agglomerate MNPs, and the latter two are beneficial for stabilizing MNPs. Surface modification is an approach that introduces protective molecules on the particle surface that increases the repulsive force between particles. It can also reduce the surface energy, improve hydrophilic/hydrophobic properties, and avoid the oxidation of Fe3O4 to g-Fe2O3. Meanwhile, by surface modification, various functional groups such as hydroxyl, carboxyl, sulfonic acid, amino, and mercapto can be introduced into the MNPs. Alternatively, various materials such as silicon dioxide, metal oxides, and quantum dots can be coated on the particle surface to adjust sorbent selectivity. Surface modification methods include physical modification and chemical bonding. The physical modification uses physical means (e.g., adsorption, coating, or wrapping) by such techniques as surface adsorption and surface deposition. The surface modification of MNPs by ultraviolet and plasma radiation is also a physical modification. The chemical modification involves a change in the surface state of MNPs

240

Solid-Phase Extraction

Figure 9.1 Different morphologies and structures for magnetic beads. (A) core-shell type, (B) multicore, (C and D) bead on bead and (E) brush (hair) morphology [15].

through chemical reactions. Fig. 9.1 illustrates common structures of chemically modified magnetic composites.

9.2.2.1

Modification with inorganic materials

Amorphous silica is insoluble in water and many other solvents but is soluble in alkali solutions and hydrofluoric acid. Its favorable chemical/thermal stability, specific composition, and physical structure result in its common use as an adsorbent. Generally, three methods are used for coating silica on the surface of MNPs, namely the St€ober reaction [16,17], embedding [18], and microemulsion [19,20]. In St€ober reaction, the silica coating is prepared by the hydrolysis of silane coupling reagents or silicates by alkali or acid catalysis. Liu et al. [21] dissolved sodium silicate in deionized water, adjusted the pH to 12e13, followed by the mixing of Fe3O4 NPs with the sodium silicate solution with ultrasonication for 30 min. The mixture was then heated to 80 C with mechanical agitation, followed by dropwise addition of hydrochloride acid to a pH of 6e7, and collection of the silica-coated MNPs. Deng et al. [22] used a sol-gel method to modify the surface of MNPs with silica by the acid-catalyzed hydrolysis of tetraethoxysilane (TEOS). In the embedding strategy, MNPs are embedded in the porous structure of silica. Wu et al. [18] mixed solid Fe(NO3)3 with ethylene glycol and spiked a porous silica substrate with this mixture, followed by heating the mixture under nitrogen, converting Fe(NO3)3 into Fe3O4 NPs in porous silica. In the microemulsion method, the formation, thickness, and morphology of silica on the surface of MNPs are controlled by the action of micelles. Ding et al. [19] prepared silica coated Fe3O4 NPs with uniform size and regular core-shell structure by the inverse microemulsion method. For obtaining a silica coating of defined thickness, the concentration of Fe3O4 NPs in the microemulsion, and the amount of added TEOS was precisely controlled.

Magnetic nanoparticle sorbents

241 NH2

OH HO

FeCl2, FeCl3

OH

TEOS

Modification HO

OH

SH

C18

NaOH

HO

OH OH

COOH

Figure 9.2 Schematic diagram of the synthesis of silica-coated MNPs modified with different functional groups [31].

MNPs with a silica coating are the most widely used intermediates for preparing sorbents with a wide range of organic surface modifications. It is often used as the protective layers of the magnetic core. Fe3O4@SiO2 NPs are easily modified by small organic molecule (e.g., bismuthiol-II [23], zincon [24], 3-mercaptopropionic acid [25], 5-sulfosalicylic acid [26]), ionic liquids [27], and polymer [28e30]) by bonding to the silica surface through reactive functional groups, Fig. 9.2. Suleiman et al. [23] employed Fe3O4@SiO2 NPs modified by bismuthiol-II for the separation and enrichment of Cr(III), Cu(II), and Pb(II) in environmental water samples. Cao et al. [26] used 5-sulfosalicylic acid modified Fe3O4@SiO2 NPs for MSPE of Se(VI), Se(IV), and two selenium amino acids in Pueraria lobata extracts. Cui et al. prepared methyl trioctyl ammonium chloride ([MTOAþ][Cl
]) ILs and chitosan modified Fe3O4@SiO2 NPs for the extraction of trace elements from biological samples [27] and inorganic Cr speciation in environmental water samples [29], respectively. In addition to silica, surface modification of MNPs with other inorganic materials, such as TiO2, Au, and Ag, have been employed. Li et al. [32] spiked Fe3O4 MNPs and chloroauric acid into sodium citrate solution at 99 C forming a wine red solution. After cooling and magnetic separation, a Fe3O4/Au nanoparticle composite was obtained with an average particle size of 35 nm. A layer of nano-gold was formed as a shell on the surface of Fe3O4 MNPs. Ma et al. [33] prepared Fe3O4@mTiO2 magnetic microspheres. First, magnetic colloidal nanocrystals clusters (MCNCs) of around 280 nm diameter consisting of nanocrystalline (7e11 nm) particles, were synthesized by the solvent-thermal method. Then TiO2 was deposited on the surface of the MCNCs by the sol-gel method and the TiO2 shell transformed into a mesoporous structure by hydrothermal treatment. Magnetic nanoparticle composite sorbents with a shell thickness of about 100 nm, saturation magnetic strength of 17.8 emu g
1, TiO2 content of 74wt%, a pore volume of 0.45 cm3 g
1 and a mesopore size of 16.4 nm were obtained.

9.2.2.2

Modification with small organic molecules

In addition to inorganic materials, MNPs can be modified with small organic molecules, such as surfactants, ionic liquids, and silane coupling reagents.

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Solid-Phase Extraction

9.2.2.2.1 Surfactant modification Surfactant modification prevents MNPs from being oxidized by air. Also, the addition of appropriate surfactants into the reaction system during synthesis is often used to adjust the size, morphology, and properties of the MNPs and to modify agglomeration. Su et al. [34] prepared Fe3O4 MNPs by coprecipitation with the addition of oleic acid resulting in oleic acid coated MNPs with improved lipophilicity. When the oleic acid was reacted with potassium permanganate, the double bond was oxidized to form carboxyl groups, improving the hydrophilicity of the MNPs. Takami et al. [35] heated a ferrous sulfate solution to 200 C in the presence of decanoic acid (C9H19COOH) and decylamine (C10H21NH2), respectively. Decanoic acid modified MNPs had an a-Fe2O3 structure with a cubic crystal shape with an average particle size of 25 nm; decylamine modified MNPs had a Fe3O4 structure with a spherical shape and an average particle size of 14 nm. These results indicate that the addition of surfactants affects the crystal shape, morphology, and size of the resultant MNPs.

9.2.2.2.2 Silane coupling reagent modification There are an abundant number of hydroxyl groups (Fe-OH) on the surface of ferrite magnetic nanoparticles suitable for reaction with silicon hydroxyl groups (Si-OH) formed by hydrolysis of a silane coupling reagent, such as TEOS, resulting in the formation of Fe-O-Si bonds. At the same time, Si-OH groups can react with each other forming Si-O-Si bonds. This process of hydrolysis and condensation proceed continuously with the formation of a silica coating on the surface of the MNPs, typically, a few nanometers thick. Besides protection from oxidation, these surface modifications can provide active sites for the introduction of other functional groups. Some silane coupling reagents containing specific functional groups (e.g., (3-mercaptopropyl)trimethoxysilane (MPTMS), 3-aminopropyltriethoxysilane (APTES), 3-(trihydroxysilyl)propyl methylphosphate) react with silica-coated MNPs installing mercapto [36e39], amino [40e44], n-octyl [45], ethylenediamine tetraacetic acid (EDTA) [46], iminodiacetic acid (IDA) [47], thiosemicarbazide [48], and phosphoric acid group [49] on the silica surface of the NPs. Most of these magnetic sorbents have good dispersibility favorable for faster extraction kinetics and facilitate selective enrichment of target compounds based on their affinity for the installed functional groups. These functional groups also allow for further modification of the particle surfaces based on their characteristic reactivity. Click reaction chemistry on mercaptosilane modified MNPs was used to obtain sulfonic acid modified MNPs [50]. The sulfonic modified MNPs exhibited enhanced adsorption capacity for Cd(II) and Pb(II) ions. Schiff base functionalization was performed on an aminosilane modified MNPs for extraction of Pb(II) and Cd(II) [51]. Chloropropyltriethoxysilane (CPTS) modified MNPs were further grafted with mono-2-ethylhexyl 2-ethylphosphonate for selective extraction of rare earth elements [52]. 3-Vinyltriethoxysilane (VTES) [53] or 3-(methacryloxy)propyltrimethoxysilane (MPS) [54] modified MNPs provided suitable substrates for attachment of imprinted polymer coatings by polymerization.

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243

9.2.2.2.3 Ionic liquid modification Ionic liquids (ILs) consist of asymmetric organic cations and inorganic/organic anion. They are characterized by low volatility, good chemical and thermal stability, and adjustable water solubility. Zhang et al. [55] spiked Fe3O4 NPs and 1-hexadecyl3-methylimidazolium bromide (C16mimBr) ILs simultaneously into an aqueous solution for the extraction of polycyclic aromatic hydrocarbons (PAHs). The adsorption of C16mimBr on the Fe3O4 NPs was driven by electrostatic interactions. The positively-charged head group of C16mimBr, an imidazole ring, strongly interacts with the negatively-charged Fe3O4 NPs. The extraction of PAHs by the ILs coated MNPs is possibly based on hydrophobic interactions and positively correlated with the alkyl chain length of the IL. Cui et al. [27] prepared methyltrioctylammonium chloride ([MTOAþ] [Cl
]) modified MNPs by mixing Fe3O4 NPs and [MTOAþ] [Cl
] under vortex for 30 min. The ILs modified MNPs were used for the extraction of metal ions. Bouri et al. [56] synthesized ILs functionalized MNPs by the reaction silica-coated Fe3O4 NPs with a silane coupling reagent containing N-methylimidazolium groups. This ILs functionalized MNPs were used for the extraction of sulfonylurea herbicides from water.

9.2.2.3

Organic polymer modification

The hydrophilic/hydrophobic properties of MNPs can be adjusted by coating with organic polymers. Modification by organic polymers increases the Zeta potential and surface charge density of MNPs stabilizing their dispersion in water/oil emulsions. A variety of organic polymers are available for the modification of MNPs, including polysaccharides (e.g., starch, cellulose, glucans, chitosan, alginate, agarose), amino acids (e.g., peptides, enzymes, proteins, gelatin), natural biological macromolecules, and polymer/copolymer composites prepared from a variety of monomers (e.g., styrene (St), acrylic acid (AA), methacrylic acid (MAA), methyl methacrylate (MMA) and acrylamide (PAM)) [12]. Organic polymer modified MNPs generally exhibit three types of structures (Fig. 9.3): (A) MNPs with a uniform coating of polymer on the surface generally referred to as the core-shell-1 structure; (B) Organic polymer wrapped around the surface of MNPs generally referred to as the core-shell-2 structure, and (C) MNPs uniformly distributed in a polymer microsphere generally referred to as the dispersion structure.

Figure 9.3 Schematic representation of different structures of magnetic polymer beads: (A) polymer coreemagnetic shell, (B) magnetic core-polymer shell, (C) magnetic particles homogeneously distributed in the polymer bead [1].

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Solid-Phase Extraction

Natural biological macromolecules can provide good biocompatibility and are typically widely available and inexpensive. Ma et al. [57] dissolved ferric and ferrous chloride in water containing sodium hydroxide under nitrogen and heated the mixture to 60 C followed by the addition of sodium alginate with stirring at 80 C to obtain alginate modified MNPs. For MNPs modified with synthetic polymers, common synthetic routes include embedding (in situ), chemical conversion, dispersion polymerization, suspension polymerization, and emulsion polymerization. All processes involve two procedures: nucleation and nuclear growth. The particle size, size distribution, molecular weight, and conversion rate are determined by the specifics of the two processes [58]. Harris et al. [59] prepared copolymer modified MNPs by the embedding method. Polyethylene oxide (PEO), 3-isocyanate methylene-l, 3, 5, 5-trimethylcyclohexyl isocyanate (IPDI), and 2, 2-dihydroxymethyl propionic acid (DMPA) were initially copolymerized. The copolymer was then dissolved in ethylene dichloride followed by mixing with MNPs for 30 min (pH 8.5e9) with stirring. After removing the excess solvent by nitrogen blowing, the pH of the magnetic suspension was adjusted by hydrochloric acid to 6.5e7. Xiong et al. [60] prepared magnetic sorbents by the in situ method. First, ferric chloride and ferrous chloride were dissolved in H2O and the mixture heated to 60 C after the addition of polyethylene glycol 4000. The mixture was adjusted to pH 11 with sodium hydroxide solution and stirred for 0.5 h to obtain PEG-4000 modified magnetic sorbents. According to XRD characterization, the particle size of the MNPs decreased from 20 to 11.5 nm after PEG addition. Liu et al. [61] used dispersion polymerization to prepare magnetic polymer microspheres by first synthesizing macromolecular polyoxyethylene monomer and then carrying out a copolymerization reaction between the monomer and styrene for 12 h at 70 C. PEG-4000 was used as the dispersive reagent and a mixture of water and ethanol as the reaction medium. Generally, the products obtained by dispersion polymerization exhibit relatively large sizes, in this case, around 11 mm. Du et al. [62] dispersed PEG-4000 modified Fe3O4 NPs in a solution of ethanol and styrene; after the addition of styrene and benzoyl peroxide (initiator), the mixture was heated for 6 h at 75 C. Polystyrene-coated MNPs were obtained with a size of about 80 nm and magnetism of 20.7 emu g
1. Liu et al. [63] used emulsion polymerization to prepare polymer-coated polymer MNPs, as shown in Fig. 9.4. First, Fe3O4 NPs of about 10 nm were synthesized by the coprecipitation method and coated with oleic acid. Then, the oleic acid coated MNPs were dispersed in a mixture of styrene, methacrylic acid (MAA), divinylbenzene (DVB), and acrylamide (AM); benzoyl peroxide

Figure 9.4 Formulation of polymer coated magnetite nanoparticles. In the first step, hydrophobic magnetite nanoparticles were produced and dispersed in styrene. In the second step, magnetic microspheres were obtained by microemulsion polymerization [63].

Magnetic nanoparticle sorbents

245

(BPO) was added as the initiator to form solution A. Hexadecanols and sodium hexadecylsulfate were dissolved in water to form Solution B. Solution A was added dropwise to solution B and after 0.5 h, ammonium persulfate (initiator) was added and the mixture stirred for a further 3 h forming Fe3O4@P(St-co-MAA-co-AM) nanoparticles. The polymer coated MNPs were of relatively large size (1e5 mm) and weak magnetic strength [47 emu g
1 (oleic acid coated MNPs) versus 1.6 emu g
1 (copolymer coated MNPs)].

9.3

Characterization methods

9.3.1

Crystal structure analysis

X-ray diffraction (XRD) is the most common technique for determination of the crystal structure of MNPs. Fe3O4 has six characteristic diffraction peaks at 2q ¼ 30.1 , 35.5 , 43.1 , 53.4 , 57.0 , and 62.6 assigned to the (220), (311), (400), (422), (511), and (440) crystal planes of Fe3O4, respectively. Cui et al. [64] characterized the crystal structure of chitosan (CTS) modified Fe3O4 NPs by XRD. As can be seen from the XRD pattern, Fig. 9.5, strong peaks at 2q ¼ 30.13 , 35.22 , 43.2 , 53.7 , 57.37 , 62.77 and 74.4 for the diffraction peaks of Fe3O4 demonstrate the existence of Fe3O4. The peak at 22.97 is attributed to an amorphous diffraction peak for SiO2, and CTS coated on Fe3O4.

9.3.2 9.3.2.1

Morphological characterization Electron microscopy

Scanning electron microscopy (SEM) uses a focused electron beam to scan the surface of the sample in a specific order of time and space to evaluate the morphology of

Fe3O4@SiO2 150

Fe3O4@SiO2@CTS 30.26 35.55

57.2

62.7

22.97

74.7

Intensity (a.u.)

53.6 43.2

100

50

0 30

60

2θ(degree)

Figure 9.5 XRD spectra of Fe3O4@SiO2 and Fe3O4@SiO2@CTS [64].

246

Solid-Phase Extraction

the MNPs. Its merits include high resolution, wide magnification range (generally 10e150,000 times), large depth of field, 3-D image capability, and simple sample preparation. Nonconductive samples need platinum(Pt)/Gold(Au) sputtering treatment of the surface before characterization. Transmission electron microscopy (TEM) provides information on the morphology, structure, and size of MNPs. When an electron beam is focused on the MNPs, some electrons pass through the samples while others are scattered. The former beams are amplified and imaged on a fluorescent screen, providing a magnification of 50e800,000 times. TEM offers high resolution and magnification but poor 3-D image capability. It is difficult to observe the real size of agglomerated particles. Li et al. [65] synthesized molecular imprinting (MIP) MNPs and characterized them by using SEM and TEM. Fig. 9.6 reveals that the uncoated, silica coated, and MIP modified MNPs had a spherical shape. Fe3O4 NPs has a uniform particle size of about 330 nm. The thickness of the silica coating on the surface of Fe3O4 is about 18 nm and the MIP coating about 22 nm.

9.3.3 9.3.3.1

Elemental analysis Electron spectroscopy

X-ray photoelectron spectroscopy (XPS) is a surface analysis technique widely used to determine the elemental composition and oxidation states of elements at the surface of MNPs by excitation of inner orbital and bonding electrons by a focussed X-ray beam. The XPS spectrum is obtained by measuring the kinetic energy and quantity of electrons. Energy dispersive spectroscopy (EDS) is a surface microanalysis technology in which nanoparticles are irradiated by an electron beam to excite characteristic X-rays. EDS provides information about the elemental analysis, chemical composition, and predicted particle structure. Fang et al. [66] synthesized a- Fe2O3@SiO2@Au and characterized the composition of the material with XPS and EDS, Fig. 9.7. The XPS spectrum contains peaks at 285.03 eV attributed to the binding energy of C1s, 103.29 eV to the binding energy of Si2p, 83.98 and 87.7 eV to the binding energy of Au4f7/2 and Au4f5/2, respectively, and 532.68 eV to the binding energy of O1s. The EDS spectrum demonstrates the existence of Fe, Si, O, Au, and Cu (Cu signal is an instrumental artifact). XPS data combined with EDS analysis is consistent with the material’s composition.

9.3.4

Surface area analysis

The Brunauer-Emmett-Teller (BET) method is commonly used for evaluating the surface area and surface morphology of MNPs by nitrogen adsorption. The larger the specific surface area, the more active sites and stronger adsorption capacity, the materials have. Ding et al. [67] synthesized mesoporous MNPs and octadecyl phosphate modified MNPs with a specific surface area and porosity distribution shown in Fig. 9.8. The N2 adsorption/desorption isotherms (Fig. 9.8A) reveals a type IV adsorption isotherm for the materials with an H3 type hysteresis loop and a

Magnetic nanoparticle sorbents

Figure 9.6 The SEM and TEM images of Fe3O4 (A), Fe3O4@SiO2 (B), and Fe3O4@SiO2-MIP (C) [65].

247

248

Solid-Phase Extraction

(A)

(B) 1800

O1s

Cu

Si

1600

Intensity (a.u.)

1400 1200

Si2p Si2s

C1s

Intensity

Au4f Au3d

1000 800

C O

600 Au

400

Fe

200 FeCu

Cu

Au

Fe

Au

0 0

200

400 600 800 Binding Energy/eV

1000

0

5

10

15

Energy (KeV)

Figure 9.7 (A) XPS spectrum of the a- Fe2O3@SiO2@Au nanocomposites. (B) EDS data from an a- Fe2O3@SiO2@Au sphere. The Cu signal is from the Cu grids of the TEM sample [66].

Figure 9.8 N2 adsorption/desorption isotherms (A) and BJH pore size distribution curve (B) of MMNPs [67].

low-pressure end of the Y-axis. Together, this indicates that the material has a strong interaction with nitrogen, and there are slit holes formed by stacking flaky particles. The pores in the materials are mesoporous.

9.3.5 9.3.5.1

Spectral analysis Fourier transform infrared spectroscopy

The Fourier transform infrared (FTIR) spectra is based on the characteristic absorption of infrared radiation due to molecular vibration and is generally nondestructive. Huang et al. [68] synthesized magnetic MOF composites and characterized them by several spectroscopic techniques. The FTIR spectra of Bi-I, Fe3O4@SiO2, Fe3O4@SiO2@ Cu(OH)2, Fe3O4@SiO2@HKUST-1, and Bi-I-functionalized Fe3O4@SiO2@ HKUST-1 are shown in Fig. 9.9. In the FTIR spectra of Fe3O4@SiO2, a broad peak around 1090 cm
1 is observed for the stretching vibration of O-Si-O demonstrating

Magnetic nanoparticle sorbents

249

Figure 9.9 (A) FTIR spectra of Bi-I, Fe3O4@SiO2, Fe3O4@SiO2@Cu(OH)2, Fe3O4@SiO2@ HKUST-1, and Bi-I-functionalizedFe3O4@SiO2@HKUST-1; (B) Fe3O4@SiO2@HKUST-1 and Bi-I-functionalized Fe3O4@SiO2@HKUST-1 with a different ratio, sample A(50 mg), B(100 mg), C(150 mg) and D(200 mg) [68].

the successful preparation of Fe3O4@SiO2. The peaks at 587, 1000 and 3576 cm
1 correspond to the Cu-O stretching, Cu-O-H bending, and O-H stretching vibrations, respectively, identified with the Cu(OH)2 coating on the Fe3O4@SiO2 nanoparticle core. For Fe3O4@SiO2@HKUST-1, the asymmetric stretching vibration at 3050 cm
1 is attributed to the aromatic ring C-H in H3BTC, absorption peaks for the C¼O bonds (1709 and 1645 cm
1) and the stretching vibration of the C¼C bonds of the benzene ring (1447 and 1374 cm
1) are also observed. The absence of Cu-O stretching at 1000 cm
1 indicates the complete in situ conversion of Cu(OH)2 into HKUST-1. In the spectrum of Bi-I-functionalized Fe3O4@SiO2@HKUST-1, a distinct peak at 1052 cm
1 is assigned to the stretching vibration of Cu-S, indicating a modification of Bi-I on Fe3O4@SiO2@HKUST-1. The vibration of S-H at 2472 cm
1 was not very strong, however. The absorption bands for Bi-Ifunctionalized Fe3O4@SiO2@HKUST-1 are different to those of Fe3O4@SiO2@ HKUST-1 in the region of 2750e4000 cm
1, clearly revealing that Bi-I molecules are grafted onto the unsaturated metal sites of the framework, rather than just adsorbed on the external surface of HKUST-1.

9.3.5.2

Absorption spectroscopy

Absorption spectroscopy is suitable for determining the properties of nanoparticles surface functionalized by organic compounds (mainly) or inorganic substrates with different oxidation states that absorb light between 200 and 800 nm. Fang et al. [66] synthesized a-Fe2O3@SiO2@Au NPs. The UV-vis spectra of their solutions, for a-Fe2O3@SiO2@Au, a- Fe2O3 and Au are shown in Fig. 9.10. As can be seen, the maximum absorption wavelengths of a- Fe2O3 and Au are 370 and 519 nm, respectively. The maximum absorption wavelengths of a-Fe2O3@SiO2@Au are at 370 and 524 nm, probably a result of a strong interaction between Au and adjacent atoms.

250

Solid-Phase Extraction

α-Fe2O3

0.6

Au

α-Fe2O3/SiO2/Au

Absorbance

0.5 0.4 0.3 Fe2O3/SiO2/Au

Fe2O3

0.2

Au 0.1 0.0 200

300

400

500

600

700

800

Wavelength (nm)

Figure 9.10 UV-vis spectra of a-Fe2O3@SiO2@Au nanocomposites, a- Fe2O3 nanocubes, and Au nanoparticles dissolved in distilled water. The inset shows a photograph of the samples [66].

9.3.6

Thermal gravimetric analysis (TGA)

Thermal gravimetric analysis (TGA) is used to investigate the relationship between the mass variation of MNPs and temperature, providing information about the coating composition. Kan et al. [69] synthesized a magnetic aspirin MIP NPs and characterized it by TGA. Fig. 9.11 shows that the amount of MIP in the magnetic sorbent accounts for 72.5% (w/w).

Figure 9.11 TGA curves of Fe3O4 (A), double-bond-functionalized Fe3O4 (B), magnetic MIPs (C) and pure MIPs (D) [69].

Magnetic nanoparticle sorbents

9.3.7

251

Zeta potential

A zetasizer can provide information about the surface charge of a material. The isoelectric point (pI) is the pH at which the surface charge of the suspended MNPs is 0. The surface charge density of the materials changes with the pH of the solution, which affects the surface adsorption of magnetic sorbents. When the pH is higher than the pI, the surface of the materials is negatively charged, and when the pH is below the pI the surface is positively charged. Zhang et al. [55] investigated the pI of Fe3O4 MNPs and reported it to be about 6.5.

9.3.8

Magnetism

A vibrating sample magnetometer (VSM) is used to investigate the saturation magnetization of MNPs, construct a magnetization curve, determine the relationship between magnetization strength and external magnetic field intensity, and to identify superparamagnetism. At room temperature, when the magnetization intensity (H) of the material is zero, the hysteresis-free magnetization curve indicates that the material is superparamagnetic, and the stationary point of the magnetization curve is the saturation magnetization strength. When a magnetic sorbent is coated with a polymer, the paramagnetism is hardly affected, but its magnetization strength will decline. Thus the core-shell structure of the material can be verified by VSM. Ma et al. [70] characterized Fe3O4@PGMA-NH2 with VSM, and the magnetization strength was found to be 16.3 emu g
1. It indicates that the nanoparticle coating did not destroy the superparamagnetism of the Fe3O4 MNPs. Luo et al. [70] synthesized Fe3O4@ SiO2/graphene and characterized its magnetization strength with VSM. Fig. 9.12 shows that the magnetization strength of Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2/graphene are 72, 43, and 21 emu g
1, respectively. It indicates that silica and graphene affect the magnetization strength of Fe3O4 MNPs but do not destroy its superparamagnetism.

Figure 9.12 Room-temperature magnetization curves of Fe3O4, Fe3O4@SiO2, and Fe3O4@ SiO2/graphene [70].

252

9.4

Solid-Phase Extraction

Magnetic carbon materials

There are many kinds of carbon nano-sorbents, including fullerenes, carbon nanotubes (CNTs), carbon nanofibers (CNFs), graphene (G), and graphene oxide (GO). They are thermally, chemically, and mechanically stable with large specific surface areas. Abundant modifiable sites facilitate easy modification. The integration of MNPs with carbon nanomaterials provides a source of carbon nanomaterials easy to separate from aqueous solution. Carbon nano-sorbents can adsorb aromatic compounds through van der Waals, hydrophobic, and/or p-p stacking interactions. For metal ions surface modification of carbon nano-sorbents is necessary to improve their adsorption capacity and selectivity.

9.4.1

Magnetic carbon nanotubes (CNTs)

Carbon nanotubes (CNTs) are usually curled by single or multiple layers of graphene to form hollow tubular structures. According to the number of graphene layers, CNTs can be divided into single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). Since pure CNTs are hydrophobic and poorly dispersed in water, it is difficult to separate and recover from the solution. The use of magnetic CNTs composites is a common method to overcome such difficulties. Magnetic CNTs preparation can be divided into two approaches, namely one-step and two-step reactions. The two-step reaction was the first approach to be used for the preparation of magnetic CNTs. In the first step, the nanoparticles are synthesized and then combined with the CNTs is a second step forming a magnetic CNTs composite. Wang et al. [71] prepared a magnetic CNTs composite by the electrostatic assembly of Fe3O4 NPs and MWCNTs. The magnetic MWCNTs were used to extract Se (IV) from water. However, the adsorbents exhibit low adsorption capacity (2.5 mg g
1) for Se (IV) with slow kinetics. To improve the adsorption performance of CNTs for metal ions strong acids (e.g., concentrated nitric acid) are often used to oxidize CNTs, resulting in abundant oxygen-containing functional groups (e.g., hydroxyl, carbonyl, and carboxyl groups) on the end and side walls of the CNTs. These functional groups assist in the adsorption of metal ions and improve the dispersion of the CNTs in solvents. These functional groups also provide abundant active sites for further modification. Sadegh et al. [72] mixed oxidized MWCNTs with Fe3O4 NPs and obtained magnetic MWCNTs with a large specific surface area (92 m2 g
1) through a simple selfassembly reaction. These magnetic CNTs exhibited an adsorption capacity of 239 mg g
1 for Hg (II). The synthesis of MNPs in a one-step process employs an in situ reaction on the surface of the CNTs. Demir et al. [73] introduced Fe3O4 NPs into MWCNTs by the one-step method. Oxidized MWCNTs were spiked into a solution of iron(II) sulfate hexahydrate in H2O: hydrazine (3:1) followed by heating under reflux for 2 h, resulting in black magnetic MWCNTs. Huiqun et al. [74] mixed MWCNTs with an acidic solution of iron nitrate with heating under reflux for 4.5 h. The subsequent addition

Magnetic nanoparticle sorbents

253

of a basic solution (NH4OH, 2.5 wt%) led to the precipitation of iron oxide onto the surface of the MWCNTs. Tarigh et al. [75] synthesized Fe3O4 NPs in situ on oxidized MWCNTs by the coprecipitation method for rapid extraction of Pb (II) and Mn (II) in the presence of the chelating agent PAN. Zhang et al. [76] prepared magnetic CNTs by a one-step solvothermal method and then grafted a sulfhydryl-containing silane reagent onto their surface. The magnetic sorbents had a large specific surface area (97 m2 g
1), and a relatively high adsorption capacity (65.4 and 65.5 mg g
1) for Pb (II) and Hg(II) ions. The magnetic composites can be further functionalized by postfunctionalization. Fayazi et al. [77] synthesized Fe3O4 NPs containing oxidized MWCNTs by the coprecipitation method subsequently modified with silane coupling reagents containing double bonds them with a double bond. The magnetic composites had good selectivity and high capacity (48.1 mg g
1) for the adsorption of Pb(II). In addition, 8-hydroxyquinoline [78] and melamine [79] functional groups were grafted on the surface of magnetic MWCNTs after the introduction of chloropropyl groups to improve the selectivity for metal ions. This in situ synthesis procedure with different iron salts is now commonly used to obtain magnetic CNTs due to its simplicity. These materials were also used for the solid-phase extraction of various organic compounds (e.g., polycyclic aromatic hydrocarbons [80] or atrazine [81]) as well as metal ions [78,79].

9.4.2

Magnetic graphene and graphene oxide adsorbents

Graphene (G) is a carbon-based material with a single-layer two-dimensional honeycomb lattice structure. Graphene is the basic framework of many graphite-based carbon materials, including CNTs and fullerenes. It has a large surface area and abundant delocalized p-electrons. It can adsorb aromatic compounds through p-p stacking. However, G contains few functional groups which can interact with metal ions; it needs appropriate surface modification [82] or addition of a chelating agent before use as a sorbent for metal ions [83]. Graphene oxide (GO) is the oxidation product of graphene. Compared with G or reduced GO, GO has more oxygen-containing groups, such as carboxyl, hydroxyl, carbonyl, and epoxy groups on its surface. It is more hydrophilic and easier to chemically modify than G-based materials. Therefore, GO is widely used for the extraction of metal ions. However, GO has good water solubility and is difficult to separate directly from solution. Magnetic GO can avoid the loss of GO during the extraction process. Magnetic graphene/graphene oxide nanoparticles can be prepared by the in situ growth of MNPs on the G/GO surface through chemical coprecipitation and hydrothermal treatments. Sun et al. [84] blended GO dispersions with a solution containing ferrous and ferric salts. Magnetic GO was obtained by coprecipitation in one step. The magnetic GO was used for the analysis of heavy metal ions in biological samples. Kazemi et al. [85] synthesized magnetic GO by a similar method and used it for the extraction of Au in environmental water samples. The adsorption capacity of this sorbent, 9.8 mg g
1, is comparatively low. To further improve the adsorption capacity of magnetic GO for metal ions, postfunctionalization is usually employed to introduce

254

Solid-Phase Extraction

additional functional groups on magnetic GO surface. Guo et al. [86] synthesized magnetic GO by a one-step coprecipitation method, then bonded 1,2-cyclohexanediamine tetraacetic acid to the GO surface using ethylenediamine. The functionalized magnetic GO has a larger specific surface area (310 m2 g
1) and a higher adsorption capacity (77.3 mg g
1 for Cr (VI) than unmodified magnetic GO and can be used for the removal of Cr (VI) from water. Cui et al. [87] synthesized magnetic GO by the solvothermal method and modified its surface with EDTA. The magnetic GO-EDTA can efficiently extract Pb, Hg, and Cu with an adsorption capacity of 508, 268, and 301 mg g
1, respectively. Amino-modified magnetic GO was prepared by the solvothermal method with the addition of diethylenetriamine [88]. Compared with the unmodified magnetic GO, the modified magnetic GO has a higher adsorption capacity (123 mg g
1) for Cr (VI). In addition, various polymers [89e91], ionic liquids [92], small molecules [93], and other functional components [94] have been used for the surface modification of GO. In the two-step reaction process, MNPs and G are mixed, and magnetic G composites (MGC) obtained by a simple ultrasonication or oscillation treatment. Since MNPs are attached to the surface of G/GO by physical adsorption, the products are not stable for reuse. Su et al. [95] prepared silica coated Fe3O4 NPs followed by the in situ polymerization of aniline monomers to obtained magnetic polyaniline (MPANI), which was mixed with a GO suspension with ultrasonication. The sorbent composite could be regenerated and reused. It was used for the preconcentration of rare earth elements in water samples based on chelation by surface functional groups on GO (hydroxyl, epoxide, carboxyl and carbonyl groups) with the rare earth elements. Coupling MNPs and G/GO via chemical bonding is expected to produce more stable products. Owing to the rich oxygen-containing functional groups on its surface GO has plenty of sites for a further chemical reaction. Generally, the MNPs are modified with silane coupling reagents [e.g., tetraethyl orthosilicate (TEOS) and (3-aminopropyl) triethoxysilane (APTES)], producing functionalized MNPs. Then, the amino-modified MNPs are attached to the GO surface by amidation reactions using crosslinking reagents, such as 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) and n-hydroxysuccinimide (NHS) [96]. These magnetic sorbents were used for the extraction of phenolic compounds from water. They can be reused at least 15 times without a significant decrease in sorption capacity.

9.4.3

Magnetic porous carbon

Typical porous carbons (PC) include activated carbon, CNFs, CNTs, G/GO and their modified materials, as well as new porous carbon materials prepared by hightemperature carbonization and template etching using different carbon-based substrates. This section mainly introduces porous carbon materials prepared by simple hightemperature carbonization process. These materials have large specific surface areas, high porosity, good thermal and chemical stability, and a simple preparation process. The sources of carbon precursors used in calcination are extensive, e.g., biological materials, polymers, CNTs and combinations of these with various templates (e.g., metal oxide and silica NPs, mesoporous silica, polymer, MOFs, etc.) [97e99].

Magnetic nanoparticle sorbents

255

Generally, magnetic porous carbon (MPC) can be prepared by mixing and calcining a magnetic source (Co, Fe3O4, Fe3O4@SiO2, etc.) and carbon source [100e102]. Habila et al. [101] used Fe3O4 NPs as a magnetic source and a mixture of glucose and polyacrylamide as a carbon source followed by a high-temperature reaction. The resultant amide modified MPC was used for MSPE of several elements from environmental water and food samples. Peng et al. [102] prepared sulfur-doped MPC by carbonization at high temperature (600 C) with Fe3O4@SiO2 as a magnetic source, sucrose as the carbon source and basic magnesium sulfate (513MOS) as template and sulfur source. During calcination, 513MOS and sucrose form sulfur ether bonds providing functional groups for selective adsorption. Magnesium oxide produced by the decomposition of 513MOS acts as a porogen and can be removed by acid washing after carbonization. It contributes to high specific surface area and porosity of the sorbents and fast adsorption/desorption kinetics in the extraction process. Due to the high S content, the MSPC possesses a high adsorption capacity for Hg(II) (343 mg g
1) with good selectivity. It was used for the analysis of Hg in environmental water samples. Another strategy is to mix magnetic metal salts and carbon precursors to simultaneously produce magnetic materials and PC after calcination. It provides a simpler way to introduce magnetic sources into PC. Wen et al. [103] used ferric chloride as a magnetic source and tea residues as a carbon source, which was mixed and dried, followed by calcination. Tea residue was carbonized to form PC while iron salt was pyrolyzed forming g-Fe2O3 on the PC, producing MPC materials used to extract As(V), Cr(V), humic acid and organic dyes from water. Chen et al. [104] prepared a magnetic nitrogen-doped mesoporous carbon material using ferric chloride as a magnetic source, mesoporous silica SBA-15 as a template, ionic liquids (1-cyanoethyl-3-methylimidazolium chloride) as carbon and nitrogen sources. After homogeneous mixing, calcination at high temperature and template removal, MPC was obtained with a mesoporous structure and a large specific surface area and pore volume. It had a good adsorption capacity, 23.6 mg g
1, for Cu(II). The MPC prepared by using mesoporous silica as a template has good porosity and can be used as a substrate for further nitrogen-doped mesoporous porous carbon materials. Yang et al. [105] used mesoporous silica SBA-15 as a template and ferric nitrate as a magnetic source. Then polyaniline (PANI) was synthesized on its surface and carbonized at a high temperature, producing nitrogen-functionalized magnetic mesoporous carbon N-Fe/OMC, Fig. 9.13. The material has a large specific surface area and good adsorption capacity of 158 and 114 mg g
1 for Pb(II) and phenol, respectively. Liu et al. [107] prepared magnetic nitrogen-sulfur codoped mesoporous carbon nanoparticles. Silica NPs modified by L-cysteine and phenolic resin as a carbon, sulfur, and nitrogen source were mixed with iron nitrate solution followed by drying and calcination to produce MPC. The codoped magnetic mesoporous carbon had good adsorption properties for Hg(II). Due to their three-dimensional network structure, MOFs can be used as a template for pore formation during high-temperature carbonization, and the organic ligands in MOFs can act as a carbon source. In recent years, many researchers have tried to prepare porous carbon using MOF as a precursor [98]. Zeolite imidazole frameworks

256

Solid-Phase Extraction

Figure 9.13 Schematic diagram of the preparation of N-Fe/OMC for the adsorption of Pb(II) and phenol [105] (A) and magnetic N-doped porous carbon by one-pot pyrolysis strategy with in situ synthesized magnetic Co nanoparticles [106] (B).

(ZIFs) have a porous network structure rich in nitrogen, which can act as a nitrogendoped carbon source, as well as Co metal that acts as a magnetic source [106,108,109]. Huang et al. [106] mixed Co(OH)2 and precursors for ZIF-8 (ZnO NPs and 2-methylimidazole) followed by calcination. During the heating process, ZnO and 2-methylimidazole form ZIF-8 and are carbonized to form nitrogen-doped PC. Co(OH)2 is pyrolyzed to form magnetic Co NPs and dispersed in porous carbon. The one-step method based on ZIF-8 is shown in Fig. 9.13. It is simple, without the need to synthesize ZIF-8 precursors or magnetic source in advance. The synthesized materials contain a large amount of pyridine and pyrrolidone nitrogen contributing to the favorable adsorption capacity, 429 mg g
1, for Hg(II). Postfunctionalization is also used to improve the selectivity of MPC for metal ions. Guo et al. [110] prepared

Magnetic nanoparticle sorbents

257

magnetic mesoporous carbon with nickel nitrate as a magnetic source, sucrose as a carbon source and mesoporous silica SBA-15 as a template. After calcination at high temperature, magnetic mesoporous carbon was obtained and then mixed with 8-hydroxyquinoline (8-HQ) solution by ultrasound, producing 8-HQ modified magnetic mesoporous carbon. The sorbent was used for the extraction of Cu(II), Pb(II), Sr(II), and Co(II) from water. Samadi et al. [111] prepared magnetic porous carbon Fe@C by one-step carbonization of ferric chloride and urea and then mixed it with 1-(2-thiazolylazo)-2-naphthol (TAN) solution under ultrasound. The TAN-modified Fe@C MPC was used for the extraction of Cu(II) in water and food samples. However, the stability and reusability of the materials obtained by physical blending were poor. At present, the most common methods for functionalizing MPC employ doping by a nitrogen/sulfur source in the calcination process. The obtained sorbents are relatively stable. In the preparation process, it is necessary to mix the source of carbon, nitrogen/sulfur, and magnetism homogeneously prior to calcination to obtain MPC with uniform nitrogen/sulfur doping and uniform distribution of MNPs.

9.5

Magnetic metal and metal oxides

Nanometer-sized metal and metal oxide particles have unique surface active sites and physicochemical properties. Coating MNPs with metal/metal oxides can provide additional active sites for adsorption or as intermediates for subsequent grafting with other functional groups or coatings. A variety of nanometer-sized metal/metal oxides have been introduced into magnetic functional sorbents, including silver, gold, zirconia (ZrO2), titanium oxide (TiO2), alumina (Al2O3), manganese oxide (MnO2), copper oxide (CuO), zinc oxide (ZnO), and magnesium oxide (MgO). Of these, ZrO2, TiO2 are usually prepared by hydrolysis of Zr or Ti containing precursors onto MNPs by the sol-gel method. Wu et al. [112] prepared Fe3O4@ZrO2 NPs by the sol-gel method. Because ZrO2 coating is negatively charged under alkaline conditions, Fe3O4@ZrO2 NPs can adsorb positively charged Cr (III), but hardly Cr (VI), in the form of 2
2
HCr2O
7 , Cr2O7 and CrO4 anions. Zhang et al. [113] prepared Fe3O4@SiO2@ TiO2 by hydrolysis of tetrabutyl titanate on a Fe3O4@SiO2 surface. The Fe3O4@ SiO2@TiO2 surface is rich in titanium hydroxyl groups (Ti-OH) and bridging hydroxyl groups. Under alkaline conditions, these oxygen-containing groups are negatively charged, which can realize the extraction of Cu(II), Cd(II), Mn(II), and Cr(III) in tap and lake water. Habila et al. [114] also prepared Fe3O4@SiO2@TiO2 by the sol-gel method. The material has a high adsorption capacity for Cu(II), Zn(II), Cd(II), and Pb(II). MNPs modified by metal oxides, such as Al2O3, MnO2, CuO, and MgO can be prepared by a one-step coprecipitation or solvothermal method with the addition of the corresponding metal salts. They were employed for the extraction of metal ions and/ or their different species [115e122]. In addition, layered dihydroxides formed by bimetallic oxides with a petal-like pore structure are efficient general adsorbents [115,117,121]. Moreover, the nanometer-sized metal/metal oxide coatings on MNPs

258

Solid-Phase Extraction

can be loaded with chelating agents or functional groups to further improve the selectivity of the sorbents. Lopez-Garcia et al. [123] prepared nano-silver coatings on Fe3O4 by in situ reduction of grafted sodium dimercaptoethanesulfonate. The materials are selective to Sb(III) and total Sb at different pH conditions, facilitating the speciation of Sb(III)/ (V). By a similar method, Hsu et al. [124] prepared Fe3O4@AuNPs modified with L-cysteine. The magnetic sorbents were used for the extraction of Hg(II). Yan et al. [125] modified the surface of Fe3O4@TiO2 with the organophosphorus extractant P204 [di(2-ethylhexyl) phosphate]. Fe3O4@TiO2@P204 can extract rare earth elements over a wide pH range (4e7). It was used for trace and ultratrace analysis of rare earth elements in environmental samples. Based on the positively charged surface of nano-Al2O3 under acidic condition, Karami et al. [126] adsorbed negatively charged sodium dodecyl sulfate (SDS) micelles on the surface of Fe3O4@Al2O3. This was then further modified by adsorption dithizone assisted by the preadsorbed micelles. These sorbents were used for the adsorption of Ag(I) by -SH and -NH functional groups on dithizone. Tavallali et al. [127] adsorbed a TX-114 micellar phase on the surface of Fe3O4@Al2O3 then coadsorbed 1-(2-pyridylazo)-2-naphthol (PAN). Cr(III) forms a complex with PAN is selectively retained on the sorbents, while Cr(VI) is barely adsorbed. Munonde et al. [128] modified 3-aminopropyltrimethoxysilane (AAPTS) on MNPs modified by MnO2 and Al2O3 (Fe3O4@MnO2, Al2O3). The resultant Fe3O4@MnO2,Al2O3@ NH2 can selectively adsorb Cr(VI) under acidic conditions.

9.6

Magnetic metal-organic frameworks

Metal-organic frameworks (MOFs) are a crystalline hybrid material with a porous structure formed by coordination bonds between metal ions or clusters and organic ligands. The structure of MOFs includes both inorganic and organic building units. Some common structures are illustrated in Fig. 9.14. The organic ligands, mainly include oxygen- and nitrogen-containing heterocyclic compounds. Oxygencontaining ligands are mostly carboxylic, phosphoric and sulfonic acids, while nitrogen-containing compounds are mainly cyanide, imidazole, azole, and pyridine. Compared with traditional carbon-based and zeolite materials, MOFs have several advantages. They are porous materials of relatively low density with a larger specific surface area (10,000 m2 g
1) and a higher porosity (90%). The pore size of the materials can be adjusted by varying either the metal ion or ligands. MOFs have good thermal and chemical stability. Finally, MOFs are relatively simple to prepare and easy to postfunctionalize. Magnetic metal-organic framework composites (MFCs) are composed of magnetic components (e.g., Fe3O4, g-Fe2O3, NiFe2O4, CoFe2O4 [129]) and MOFs. MFCs inherit the advantages of easy and rapid magnetic separation, diversity of MOFs components, and large specific surface area. They have been widely used in catalysis, drug controlled release, biomedical analysis, and as adsorbents in complex matrices (Table 9.2). Various synthetic strategies have been used to prepare MFCs and can

Magnetic nanoparticle sorbents

259

Figure 9.14 Different types of commonly used MOFs.

be divided into the following categories: embedding method, encapsulation method, layer-by-layer self-assembly, physical mixing, postfunctionalization, and other preparation methods.

9.6.1

Embedding method

In this method, MNPs are embedded into the framework of MOFs retaining the original crystal morphology of MOFs. The commonly used preparation method is to add preprepared MNPs into a solution of MOFs precursor containing inorganic metal salts and organic ligands with ultrasound agitation. The MOFs are grown in situ by the hydrothermal or solvothermal methods. The morphology of MFCs synthesized by this method is basically the same as the pure MOFs without MNPs addition. Because of its relatively simple operation, this method can be used to prepare a wide variety of magnetic composites with different MOFs. Ke et al. [133] used Fe3O4 nanorods as a magnetic source for the preparation of Fe3O4 nanoparticles intercalated with HKUST-1. Doherty et al. [138] placed the easily prepared magnetic fibers CoFe2O4 or NiFe2O4 in the precursor reaction solution for MOF-5. After the hydrothermal reaction, the magnetic fibers with MOF-5-like morphology were obtained. The MFCs was used for the extraction of polycyclic aromatic hydrocarbons (PAHs) from aqueous solution.

9.6.2

Encapsulation method

The encapsulation method is used to encapsulate the magnetic components in the MOFs structure. Typically, a layer of organic polymer or carboxylic acid, compatible with MOFs, is modified on the surface of the magnetic components; then the modified material is added to the precursor solution for the MOFs synthesis, resulting in the growth of the MOFs on the surface of the magnetic components controlled by the

260

Table 9.2 Some MFCs and their applications. Magnetic component

MOFs

Metal

Organic linkers

Application

Reference

ɤ-Fe2O3

DUT HKUST-1

Cu2þ or Al3þ

Fe3O4 Fe3O4 Fe3O4 Fe3O4 Fe3O4-Pyridine)/

Cu- carboxylate MOF ZIF-8 HKUST-1 UiO-66 Cu3(BTC)2

H2NDC H2BPDC H3BTC

Catalysis

[130]

2þ

H2BPDCc

Catalysis

[131]

2þ

2-HmIm

Catalysis

[132]

2þ

H3BTC

Drug delivery

[133]

H2BDC

Phosphoproteome research

[134]

Cu Zn

Cu

4þ

Zr

2þ

Cu

H3BTC

Pd

2þ

preconcentration

[135]

2þ

detection

[136]

Cu3(BTC)2

Cu

H3BTC

Pb

Fe3O4/MIL-53(Al)

MIL-53(Al)

Al3þ

H2BDC

Pb2þ removal

[137]

2þ

H2BDC

PAH sequestration

[138]

3þ

H2BDC

PAH sequestration

[139]

2þ

H3BTC

Dye removal

[140]

CoFe2O4 Or NiFe2O4 Fe3O4 Fe3O4 Cu3(BTC)2

MOF-5 MIL-101 Cu3(BTC)2

Zn Cr

Cu

Solid-Phase Extraction

Fe3O4/Cu3(BTC)2eH2Dz

2þ

Magnetic nanoparticle sorbents

261

Figure 9.15 Scheme for the controlled encapsulation of nanoparticles in ZIF-8 crystals [141].

surface modifier groups, and finally the magnetic components are encapsulated in the interior of MOFs structure. By using this method, MFCs with core-shell structures are easily obtained. Lu et al. [141] used PVP-modified NPs as the core, and encapsulated a variety of NPs into ZIF-8 by in situ growth, based on the strong interaction between PVP and Zn2þ, Fig. 9.15. Zhang et al. [132] formed a layer of sodium polystyrene sulfonate on the surface of Fe3O4 NPs, which was then added to the precursor solution for ZIF-8; the Fe3O4@ZIF-8 composite with a core-shell structure was obtained by in situ growth. Zhao et al. [134] coated Fe3O4 NPs with a layer of polydopamine (PDA) based on self-polymerization of dopamine in alkaline buffer solution; then Fe3O4@PDA was dispersed in the precursor reaction solution of UiO-66, and the in situ growth of UiO-66 was realized by the strong interaction between PDA and Zr4þ. A similar method was used to prepare other MFCs with different core-shell structures, such as Fe3O4@PDA@[Cu3(btc)2] [142].

9.6.3

Layer-by-layer self-assembly

Layer-by-layer self-assembly (also known as liquid phase epitaxy) uses MNPs modified with specific functional groups, which are added to a solution containing metal ions or organic ligands in a repetitive process to realize the layer-by-layer growth of MOFs and finally form MFCs. This method was first proposed by Fisher and Kitagawa’s group [143e147] for the preparation of thin films of MOFs. The substrate was modified by molecules containing carboxyl, mercapto or amino functional groups; then sequential deposits of thin films of MOFs were deposited on the substrate from a solution of metal sources and organic ligands through molecular self-assembly. The MFCs prepared by this method maintain the original morphology of MNPs while

262

Solid-Phase Extraction

acquiring a core-shell structure. In 2012, Ke et al. [148] prepared MFCs with a core-shell structure, Fe3O4@[Cu3(BTC)2] and Fe3O4@MIL-100(Fe), by using thioglycolic acid to modify the surface of the MNPs.

9.6.4

Physical mixing

In this method, ultrasound is used to mix solutions containing MNPs and MOFs. This method requires the prior preparation of the magnetic and MOFs components. The key point is that there should be a strong interaction between the magnetic and MOFs components, such as electrostatic interactions, to ensure the stability of the MFCs and to meet the requirements for practical application. In 2012, Yan et al. proposed the mixing method strategy for the preparation of MFCs [139]. Fe3O4@ SiO2 NPs and MIL-101 were mixed in aqueous solution under ultrasound; based on the strong electrostatic interaction between negatively charged Fe3O4@SiO2 and MIL-101 with positive surface charge, Fe3O4@SiO2-MIL-101 composites were obtained. These MFCs were used for the extraction of PAHs from aqueous solution. Currently, the physical mixing method, as a simple, fast, and efficient preparation technology, has been widely used [149e152].

9.6.5

Postmagnetization

This method involves an in situ growth of magnetic particles on the surface of MOFs for the preparation of MFCs. Usually, the preprepared MOFs materials are added to the precursor solution of Fe3O4, and then the magnetic components are loaded on to the MOFs by in situ precipitation. Saikia et al. [153] synthesized MIL-101 (Cr) which was then dispersed in an aqueous solution of FeCl3 and FeCl2; the Fe3O4 NPs were synthesized in situ on the MOFs by coprecipitation, producing MFCs of Fe3O4@ MIL-101. By using a similar strategy, Jabbari et al. [154] prepared magnetic MOF@GO ternary composites with good adsorption properties for the adsorption of methylene blue in aqueous solution, Fig. 9.16.

9.6.6

Other methods

In recent years, some new synthetic strategies have been developed for the preparation of MFCs. Panda et al. [155] used MIL-53 (Fe) as a template and converted part of its skeleton structure into Fe2O3 by heating, and Fe2O3/MIL-53(Fe) composites with good magnetic properties were obtained. During the preparation of nano-Fe-MOFs by using different organic reagents as reaction solvents, Sethi et al. [156] found that the magnetic properties of the products obtained in different solvents differed greatly, although the nano-Fe-MOFs have a similar crystal structure to MIL-88B. When dimethyl sulfoxide was used as the reaction solvent, the MOFs had no magnetism, but with N,N-dimethylformamide good magnetism (up to 50 emu g
1) was obtained. Fan et al. [157] prepared a core-shell MFCs, Cu-CuFe2O4@HKUST-1, by using

Magnetic nanoparticle sorbents

263

Figure 9.16 Schematic view of the formation of Cu-BTC MOF and Fe3O4 MNPs on the GO layer [154].

Cu-CuFe2O4 as the magnetic source for MFCs and metal source for the MOFs. When H3BTC solution was added into the dispersed solution of the Cu-CuFe2O4 MNPs, the Cu(0) component in the Cu-CuFe2O4 MNPs produced Cu2þ inducing MOF nucleation on the Cu-CuFe2O4 MNPs surface. With the consumption of the Cu(0) component and the crystallization of HKUST-1, the Cu-CuFe2O4 core was gradually covered by the HKUST-1 crystal shell. Overall, although progress has been made in the preparation of MFCs materials, there are still some problems in need of an urgent solution. (1) When MFCs are prepared by the embedding or mixing methods, the materials are often doped with unmagnetized MOFs or magnetic particles, which are not functionalized by MOFs, which are difficult to purify; (2) For MFCs prepared by encapsulation or layer-by-layer self-assembly methods, it is usually necessary to prefunctionalize the surface of MNPs with certain functional groups, requiring additional preparation steps; (3) MFCs prepared by physical mixing rely on electrostatic, hydrogen bonding, or van der Waals interactions between the magnetic particles and MOFs, which are relatively unstable in applications requiring harsh conditions; (4) In postmagnetization methods, the MNPs grown in situ block the MOFs pore structure, resulting in a lower specific surface area (in addition the MNPs growing in situ are bare without any protection and easily oxidized or corroded in acid solution resulting in poor stability); (5) it is difficult to achieve large-scale production of MFCs by the current synthetic methods.

264

9.7

Solid-Phase Extraction

Magnetic porous organic polymers

Porous organic polymers (POPs) are porous network polymer materials prepared by covalent bonding of organic monomers composed of lightweight elements (e.g., C, H, O, N, B). According to the structure and crystalline characteristics, POPs can be divided into crystalline covalent organic frameworks (COFs) and amorphous hypercrosslinked polymers (HCPs), intrinsic microporous polymers (PIMs), and conjugated microporous polymers (CMPs). There are also some POPs based on special structures such as Schiff base network polymers (SNWs), covalent triazine frameworks (CTFs), and porous aromatic frameworks (PAFs). Compared with inorganic porous materials, POPs have good stability, large specific surface areas, high porosity, low skeleton density, more adsorption sites per unit mass, and no metal components. However, POPs are difficult to separate and recover because of their lightweight, and many POPs use organic ligands with an aromatic structure to form hydrophobic conjugated networks with poor dispersion properties making their use for extraction somewhat difficult. The combination of POPs and MNPs with good dispersibility and magnetism endows the magnetic materials with abundant pore and adsorption sites and favorable properties for applications in extraction. Typical preparation method for magnetic POPs (MOPs) can be categorized as (i) physical mixing; (ii) one pot, simultaneous synthesis of POPs and MNPs; (iii) in situ growth of POPs on MNPs; (iv) post magnetization in situ growth of MNPs on POPs; and (v) chemical bridging.

9.7.1

Physical mixing

The preparation of MOPs by physical mixing relies on physical interactions between POPs and MNPs. Although this method is simple and easy to operate, the instability of MOPs often affects the extraction reproducibility. Hypercrosslinked polystyrene (HCP) and Fe3O4 NPs were mixed by vortex in methanol and assembled spontaneously into HCP/Fe3O4 nanoparticles isolated by a magnet, Fig. 9.17. The HCP/ Fe3O4 NPs were used for the extraction of four sulfonamides from water and milk samples [158].

Figure 9.17 Synthesis of hypercrosslinked polystyrene Fe3O4 NPs [158].

Magnetic nanoparticle sorbents

265

Figure 9.18 Synthesis of the CTF/Fe2O3 composite by the microwave-enhanced hightemperature ionothermal method [159].

9.7.2

One-pot synthesis

The one-pot synthesis of MOPs requires the simultaneous synthesis of the POPs and MNPs. Although a few steps are involved, the reaction conditions for POPs and MNPs must be compatible, which significantly limits the scope of this approach. Zhang et al. [159] spiked FeCl3$6H2O into a mixture of zinc chloride and terephthalonitrile to form amorphous CTF/Fe2O3 nanoparticles with microwave assistance, Fig. 9.18. The CTF/Fe2O3 composites had high surface areas (930e1149 m2 g
1) and a saturation magnetization at 300K from 1.1 to 5.9 emu g
1 depending on the Fe2O3 content (6.43e12.4wt%). These composites were used for the extraction of methyl orange from aqueous solution. Ren et al. [160] prepared CTF/Fe2O3 composites by a similar method for the extraction of perfluorinated compounds from environmental water samples. Under the optimum conditions, this method has a lower detection limit (0.62e1.39 ng/L).

9.7.3

In situ growth synthesis

In this strategy, MOPs are synthesized by adding the MNPs to the monomers for the POP synthesis with a coating of the MNPs with the POPs; POPs monomers need to be modified for the subsequent POPs growth on the MNPs. To avoid damaging the MNPs during the synthesis procedure, this strategy is used only for the magnetization of those POPs, which can be obtained under mild condition, e.g., TpPa-1 synthesized at room temperature [161]. 1,3,5-Triformylphoroglucinol (Tp) was first bonded onto the amino-functionalized MNPs and then added into a solution containing Tp and p-phenylenediamine (Pa-1). After mechanical agitation at room temperature for 30 min, the brown colored TpPa-1 was magnetically isolated and washed with N,N-dimethylformamide, Fig. 9.19. Huang et al. [162] synthesized POPs directly on the surface of Fe3O4@SiO2 under mild reaction conditions forming MNPs encapsulated with POPs, Fig. 9.20. The synthesis involves a 12 h reaction in water at ice-bath temperature, avoiding the use of organic solvents and high-temperature reaction conditions. The MOPs have a moderate specific surface area (270 m2 g
1) and an adsorption capacity for Hg(II)

266

Figure 9.19 Synthesis and Application of the magnetic TpPa-1 Sorbent [161]. Solid-Phase Extraction

Magnetic nanoparticle sorbents

Figure 9.20 Direct synthesis of POPs on the Fe3O4@SiO2 MNPs [162] (A) and the linkage strategy for the synthesis of PP-CMP on the APB modified MNPs [163] (B).

267

268

Solid-Phase Extraction

up to 703 mg g
1. Zhou et al. [163] synthesized polyphenylene conjugated microporous polymer (PP-CMP) on the surface of MNPs through the coupling reaction of phenylboronic acid and bromobenzene monomers. Specifically, aminophenylboric acid (APB)-modified MNPs were added to a solution of terephthalic acid and 1,2,4,5-tetrabromobenzene, Fig. 9.20. The PP-CMP magnetic sorbent was used for the extraction of PAHs in human urine.

9.7.4

Postmagnetization and chemical bridging

This method is simple and universal, avoiding the destruction of MNPs under the harsh conditions for POPs synthesis. Rengaraj et al. [164] synthesized Fe3O4 NPs on a covalent triazine polymer (CTPs) by adding ferrous and ferric salts to the preprepared CTPs solution. The magnetic CTPs were used for the selective extraction of Sr(II) from seawater with an adsorption capacity of 128 mg g
1. Wang et al. [165] prepared Fe3O4 NPs on a porphyrin-based porous organic polymer by the coprecipitation method. The magnetic porphyrin-based porous organic polymer (MP-POP) was used for the extraction of benzoyl urea insecticides from vegetables. The adsorption capacity for the target analytes was 1.90e2.00 mg g
1. Yan et al. [166] reduced nickel ions in situ on CTFs by a high-temperature hydrothermal method (Fig. 9.19A). The magnetic CTFs/Ni composites exhibit good extraction performance for phthalate esters in plastic packaging materials. Chemical bridging offers an alternative strategy to connect preprepared MNPs and POPs. Huang et al. [167] prepared amino-modified Fe3O4 NPs and N and S-rich POPs, polythiocyanate polymer (PTMT), respectively. The PTMT was connected to the surface of the MNPs by crosslinking of glutaraldehyde, producing magnetic Fe3O4@PTMT. The sorbent exhibited good adsorption performance for Hg(II), Pb(II), and Cd(II), with an adsorption capacity of 217e603 mg g
1 and fast adsorption kinetics. Additional applications are given in [209e213].

9.8 9.8.1

Others magnetic nanoparticle sorbents Magnetic mesoporous materials

Mesoporous materials are porous materials with pore diameters ranging from 2 to 50 nm. They have large specific surface areas, adjustable mesoporous channels, large pore volume, and easy surface modification. Magnetic mesoporous materials have demonstrated a good potential for sample pretreatment, such as the extraction of metal ions, adsorption of organic compounds, and the selective enrichment of proteins or peptides [168]. Mesoporous silica gel is a commonly used mesoporous material because of its simple preparation and low cost. It is often used in combination with MNPs to prepare magnetic mesoporous silica. The specific preparation process involves the sol-gel reaction on the surface of MNPs, in the presence of a porogen/template (e.g., surfactant micelle template), and subsequent template removal. Due to the variety of silane

Magnetic nanoparticle sorbents

269

coupling reagents, different functional groups (e.g., sulfhydryl, amino, IDA) can be introduced into the mesoporous silica coatings. Li et al. [169] synthesized mercaptosilane modified magnetic mesoporous silica for the extraction of Hg(II) and Pb(II) from water. Yuan et al. [170] prepared an aminosilane-modified magnetic mesoporous silica, Fe3O4@SiO2@meso-SiO2-NH2, Fig. 9.21, suitable for the extraction of Pb(II), Cu(II), and Cd(II). Zhang et al. [172] prepared IDA modified magnetic mesoporous silica for the extraction of Cd(II), Mn(II), and Pb(II) from environmental water and biological samples. Zhao et al. [171] synthesized cyanosilane modified magnetic mesoporous silica (MMS-AN), and then ammoniated it to obtain ammonia oxime modified composite (MMS-AO), Fig. 9.21. Compared with the unmodified sorbents, MMS-AO exhibited high adsorption capacity for U(VI) (277 mg g
1). Zhang et al. [173] immobilized a polyethylene imide polymer on the surface of magnetic mesoporous silica. The sorbents had good adsorption properties for Ag NPs with an adsorption capacity up to 909 mg g
1. Magnetic mesoporous TiO2 is often used for extraction of phosphorylated peptides [33,174] and metal ions [175,176]. Zhao et al. [176] prepared mesoporous TiO2 on the surface of MNPs coated with a phenolic resin coated. The magnetic mesoporous TiO2 was used for the extraction As (V) from the water with an adsorption capacity of 139 mg g
1. Because of the good compatibility between titanium and silica sols, TiO2 coating can be modified on the surface of magnetic mesoporous silica. These sorbents are commonly used for the extraction of phosphorylated peptides [177].

9.8.2

Magnetic molecular/ion imprinting polymer

Molecular/Ion Imprinted Polymers (MIPs/IIPs) are novel adsorbents with specific recognition capability for template molecules or ions prepared by imprinting technology. Under prescribed reaction conditions, monomers, crosslinking reagents, initiators and template molecules or ions form highly crosslinked three-dimensional network polymers. Removal of the template molecules or ions leaves a cavity in the polymer structure, which matches the size, spatial structure, and binding sites of the template. MIPs/IIPs have a high selectivity for the template molecules or ions. They are relatively easy to prepare and stable and with many reported applications. However, repeated filtration and centrifugation steps are often required for phase separation and recovery of products in extraction studies. These operations are not conducive to their rapid preparation and separation. The combination of magnetic materials and MIPs/IIPs is a good way to solve this problem. Bulk polymerization is the most common and method for their preparation. Typically weak polar solvents are used to dissolve template molecules and functional monomers with the subsequent addition of crosslinking reagents and an initiator. After ultrasonic degassing, the polymerization reaction is initiated by thermal initiation or UV irradiation for the required time. The polymer is crushed, ground, screened and washed to remove template molecules. This method is time-consuming and laborious, cannot support large-scale production, and the product particles are not uniform. Sorribes-Soriano et al. [178] prepared magnetic MIPs (MMIPs) for cocaine by bulk polymerization in the presence of MNPs. Polyethylene glycol (PEG) and

270

Figure 9.21 Synthesis of Fe3O4@SiO2@meso-SiO2-NH2 microsphere and its use for the extraction of heavy metal ions [170] (A) and schematic illustration of the preparation procedure of MMS-AO [171] (B). Solid-Phase Extraction

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3-(trimethoxysilyl)propyl methacrylate (V) were used for surface modification of the MNPs. A mixture of cocaine (template), MAA (monomer), EDMA (crosslinker), AIBN (initiator), and modified MNPs were dispersed in acetonitrile (porogen), and the polymerization carried out at 60 C for 24 h with magnetic stirring. The MMIPs were subsequently crushed in an agate mortar, dried at 80 C overnight, and sieved to obtain a homogeneous particle size. In conventional molecular imprinting, there are still several problems such as incomplete template removal, low binding capacity, poor site accessibility, and slow mass transfer. Surface imprinting techniques have been proposed as a way to overcome these limitations by localizing the cavities close to the polymer’s surface. At present, magnetic MIPs/IIPs are mostly prepared by surface imprinting technology, since surface imprinting can localize imprinting sites at the surface of MNPs. These materials have good selectivity and fast adsorption kinetics for template molecules and ions. Ming et al. [179] prepared core-shell structured composite of Fe3O4 NPs and MIPs by a surface imprinting technique in combination with precipitation polymerization for the selective detection of 17b-estradiol (17b-E2). The detection was based on the competitive desorption of fluorescein from the magnetic MIPs. The competitive rebinding of 17b-E2 to the corresponding recognition sites regulated the release of fluorescein and resulted in an enhanced fluorescence signal. Li et al. [180] proposed a protocol for the preparation of glycoproteins-imprinted MNPs by boronate affinity-based controllable oriented surface imprinting, Fig. 9.22. Self-polymerization of 2-anilinoethanol was used to form an imprinting coating of appropriate thickness. This imprinting protocol allows for nearly complete removal of template molecules and provided good accessibility to target molecules, which is especially suitable for the imprinting of proteins. In addition, excessive binding sites on the boronic acid functionalized MNPs are almost completely covered by the imprinting coating, which effectively suppresses non-specific adsorption. Luo et al. [181] prepared magnetic Cu(II) IIP by a combination of surface imprinting technology and the sol-gel method. Since the imprinted sites are on the surface of MNPs, the prepared materials have fast adsorption kinetics and higher selectivity for Cu(II) over nonimprinted sorbents. The selectivity coefficients are 29.2 and 38.2 for Cu(II)/Zn(II) and Cu(II)/Ni (Cu), respectively, and the adsorption capacity of Cu(II) is 24.2 mg g
1, which is higher than for nonimprinted materials (5.2 mg g
1). The magnetic Cu(II)-IIP was used for the selective extraction of trace of Cu(II) in water samples [182]. Hierarchical imprinting (also known as double imprinting) can improve mass transfer kinetics of imprinting materials, facilitate template elution, and improve the leakage of target compounds to some extent. In this technology, sacrificial templates of different sizes are often used in the imprinting reactions. Removing sacrificial templates can leave relatively large holes in imprinted materials, which facilitates the rapid mass transfer of template molecules/ions. Cen et al. [183] used Cd(II) and CTAB surfactant micelles as templates and aminosilane and TEOS as functional monomers and crosslinking reagents, respectively, in a sol-gel polymerization reaction carried out in the presence of Fe3O4 NPs. Magnetic mesoporous silica Cd(II) IIPs were obtained. There are large holes left by the micelle template around the ion

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Figure 9.22 Synthesis of (A) DFFPBA-MNPs and (B) glycoprotein-imprinted MNPs using oriented surface imprinting [180].

imprinting sites, which is beneficial for the rapid mass transfer of the template Cd(II) ion. Wang et al. [184] prepared a double-template MIP on the surface of magnetic mesoporous silica (Fe3O4@mSiO2@DT-MIP) using dibenzothiophene (DBT) and 4-methyldibenzothiophene (4-MDBT) as template molecules. The adsorption capacity of the magnetic sorbents for DBT and 4-MDBT reached 104.2 and 113.6 mg g
1, respectively. Fe3O4@mSiO2@DT-MIP was used for the selective and simultaneous removal of dibenzothiophene (DBT) and 4-methyldibenzothiophene (4-MDBT) from gasoline. Wei et al. [185] prepared double imprinting magnetic materials with Pb(II) and Cd(II) as the templates. This material demonstrated favorable selectivity for both template ions and a high adsorption capacity. It was used for the selective extraction of Pb(II) and Cd(II) from food and environmental samples. In the template inspired polymer preparation process, even after repeated washing, not all of the template may be removed. When imprinted materials are used for extraction the residual template may gradually be released, resulting in leakage that affects the accuracy by which the concentration of the template can be determined. Dummy template can be used to prepare imprinted materials, which can effectively avoid the

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leakage problem. A substance with a similar structure to the target analytes is selected as the template, such as an analog. Due to the small radius of metal ions and the small differences in radius between metal ions, the dummy template method is used quite commonly. Bagheri et al. [186] prepared magnetic dummy (D) MIPs with propanamide as a dummy template molecule for the extraction of acrylamide in biscuit samples. The preparation of the magnetic sorbent involved encapsulation of Fe3O4 NPs by anchoring hydrophilic PEG chains on the surface of the NPs then polymerization and imprinting of chitosan on the modified Fe3O4 NPs surface to produce specific molecular binding sites. The selectivity of magnetic DMIP was evaluated by comparing the adsorption capacity, imprinting factors (IF) and selectivity factors (SF) for acrylamide and similar compounds. The IF values for acrylamide, propanamide, L-asparagine, 6-aminocaproic acid, and N-tert-butylacrylamide were 3.49, 3.54, 2.14, 1.73 and 1.5, respectively, and the SF of four analogues 0.90, 1.67, 2.13 and 2.48, respectively. Luminescent nanoparticles (NPs) incorporated in MIPs have shown great potential as sensors for a wide range of applications. Luo et al. [187] synthesized a magnetic imprinting material (Fe3O4@CdTe-IIP) by preparing CdTe quantum dots (QDs) on the MNPs surface and then eluting Cd(II) with EDTA. The Fe3O4@CdTe-IIP sorbent had good selectivity for Cd(II). When Cd(II) was adsorbed on MNPs surface, the fluorescence of CdTe QDs gradually recovers and is enhanced when Cd(II) is combined with Te(II). It can be used as a visual monitor for the real-time adsorption of Cd(II) on materials. Li et al. [188] fabricated multifunctional magnetic luminescent MIP nanocomposites via a one-pot emulsion reaction using polystyrene-comethacrylic acid polymer, Fe3O4 NPs, and luminescent LaVO4:Eu3þ NPs as building blocks with a phenanthrene template. The resulting nanocomposites were used for the luminescence detection of phenanthrene.

9.8.3

Magnetic restricted access materials

Restricted access materials (RAM) are biocompatible sorbents with a physical or chemical diffusion barrier. They limit access of macromolecules such as proteins to the inner surface of porous materials by either size exclusion (physical barrier dependent on the pore structure) and or hydrophilicity (chemical barrier dependent on the formation of a surface hydrophilic layer). Small target compounds can be selectively retained by the sorbent by their transport through the pore structure and favorable interactions with sorption sites on the inner surface of the material. RAM materials avoid the need for protein precipitation when working with biological fluids, which can easily cause the loss of analytes. They can be used directly for sample treatment after simple dilution, saving time, and simplifying the treatment steps [189]. Combining RAM with MNPs, the resultant restricted accessed MNPs can be used to analyze biological samples with complex matrixes (such as urine and serum). The installation of a porous hydrophilic layer on the surface of some adsorbents (e.g., silica, CNTs, magnetic materials, etc.) is a common method to prepare RAM materials. Ye et al. [190] immobilized a nonionic surfactant TW20 on the surface of a C12 functionalized Fe3O4 NPs to extract steroid hormones from environmental water

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Solid-Phase Extraction

and human urine samples. Yan et al. [191] synthesized a restricted accessed MNPs Fe3O4@SiO2@PAR@TW20 for the extraction of metal ions from biological fluids. The magnetic RAM material was coated by TW20 and modified by 4-2-pyridylazoresorcinol (PAR) on the inner surface. The magnetic RAM sorbent was used for the extraction of Cd(II), Pb(II), Cr(III), La(III), and Nd(II) from human serum and urine samples. Nanoparticle coatings containing mesoporous silica can provide mesoporous channels for restricted access materials as well as introducing functional groups into those channels. Liu et al. [192] modified the surface of Fe3O4 NPs with mesoporous silica coatings containing C18 groups as an external RAM barrier. The magnetic mesoporous RAM materials were used for the extraction of small molecules from mouse serum. The combination of RAM and imprinted materials can further improve the selectivity toward target compounds. Lv et al. [193] prepared a magnetic microsphere restricted access media-molecularly imprinted (RAM-MIMM) material through layer by layer modification.

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Metal-organic frameworks

10

Gongke Li, Ling Xia, Jianwei Dong, Yanlong Chen, Yanxia Li School of Chemistry, Sun Yat-sen University, Guangzhou, Guangdong, PR China

10.1

Introduction

Porous metal-organic frameworks (MOFs), are a type of adsorption materials, formed from inorganic metals and multidentate organic ligands through coordination bonds, as a crystalline network [1,2]. The combination of inorganic and organic units in their construction results in favorable properties, such as high porosity and high specific surface area for use as selective adsorbents. The porosity of MOFs may reach 90% of their volume with specific surface areas up to over 7000 m2/g [3,4]. Another feature of MOFs is the designable structure. First, various inorganic metal and organic ligands can be chosen for producing MOFs for specific applications [5]. The pore size can be adjusted by varying the metal and ligands, as well as by controlling the reaction conditions [6]. In addition, MOFs can be further functionalized to meet the requirements of specific applications. By post-synthetic modification, functional groups with different properties can be introduced [7,8]. These characteristics allow MOFs to be utilized as a sorbent for solid-phase extraction. MOFs are typically formed by self-assembly of metal ions and ligands in a one-pot reaction [9]. During the reaction, elevated temperature and pressures speed up product formation at the expense of single crystals growth. Rate-controlled reactions under mild conditions are preferred to obtain crystalline materials. Among the various possible synthetic methods, solvothermal/hydrothermal synthesis is the most widely used. In this method, ligands are mixed with metal ions or clusters in a solvent, encapsulated in a Teflon-lined bomb, and heated to a defined temperature [10]. Coordination reactions occur under conditions of high temperature and pressure. This method has several advantages, including perfect crystalline formation, simple operation, and low energy consumption [11]. Most of the well known MOFs, for example, the isoreticular metal-organic framework (IRMOF) series, materials of Institute Lavoisier (MIL) series, zeolitic imidazolate framework (ZIF) series, and University of Oslo (UiO) series, are synthesized by this method [12]. Alternative synthetic methods to prepare MOFs for SPE include the microwave/ultrasonic assisted method and the electrochemical method. Microwave/ultrasonic assisted synthesis method takes advantage of field assisted technology to accelerate coordination reactions reducing reaction times to minutes, instead of several days, typical of solvothermal methods [13]. For electrochemical synthesis, a metal plate is immersed in a solution of the organic ligands and used as an anode in an electrochemical cell. Application of a voltage results in the formation of a layer of the MOF on the cathode with reaction times typically less than 1 h [14].

Solid-Phase Extraction. https://doi.org/10.1016/B978-0-12-816906-3.00010-8 Copyright © 2020 Elsevier Inc. All rights reserved.

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Since the first application of MOFs in analytical chemistry by Yan’s group [15], their applications have grown to include sampling, solid-phase extraction, gas chromatography, and high-performance liquid chromatography. For solid-phase extraction, four series of MOFs have been used as sorbets, the IRMOF series, MIL series, ZIF series, and UiO series. Although high-throughput synthesis strategies for MOFs catch much attention [16], the amount typically used as sorbents for SPE has remained small. This favors their use in miniaturized SPE formats, including micro-solid-phase extraction (mSPE), solid-phase microextraction (SPME), dispersive solid-phase extraction (DSPE), and magnetic solid-phase extraction (MSPE), Fig. 10.1.

10.2

IRMOF in solid-phase extraction

The isoreticular metal-organic framework (IRMOF) series, are self-assembled from an inorganic unit [Zn4O]6þ and various aromatic carboxylic acid ligands to form octahedral crystalline structures [17]. The IRMOF series materials have the same topological structure but various pore sizes created by the use of different dicarboxylic acid ligands. Having a designable uniform structure with high porosity, they are suitable sorbents for SPE. Li’s group described the use of an MOF-5 bar for direct extraction of volatile sulfides from plants [18]. However, the lack of water stability of IRMOF series prevents their application as a sorbent for extraction from aqueous samples. To enhance the stability of IRMOF in an aqueous environment, combining MOFs with hydrophobic “shield” materials is a common approach [19]. For example, carbon nanotubes(CNTs) hybrid MOF-199 was successfully applied for the noninvasive analysis of trace ethylene, methanol, and ethanol from fruit samples with relatively high humidity [20]. Carbonization can also be used to enhance the water stability of IRMOF series material and expand its application in an aqueous sample [21]. Due to the moisture-sensitive nature of IRMOF series materials, they are usually composited with other substrate materials to form the sorbent for SPE. As shown in

Figure 10.1 Scheme of metal-organic frameworks application in solid-phase extraction.

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Table 10.1 Application of IRMOFs in SPE. MOFs-based sorbent

Analysis methods

Analyte

Sample matrix

MOF-5, MOF-199

VA-D-SPMEHPLC

Parabens

Water, urine, cosmetics

[22]

MOF-5/PAN

SPE-HPLC

Drugs

Urine

[23]

Cu/MOF-5

HS-SPME-GC/MS

Volatile sulfides

Plants

[18]

Fe3O4/MOF-5

MSPE-LC-MS/MS or GC/MS

PAHs, pesticide

Soil, food, plant

[24]

Fe3O4/MOF-5

MSPE-HPLC

Pesticides

Water

[25]

Fe/SiO2/MOF-5

MSPE-HPLC

PAHs

Water

[26]

Fe3O4/MOF-5-C

MSPE-HPLC

Chlorophenols

Mushroom

[21]

Fe3O4/IRMOF-3

MSPE-AAS

Metal ions

Water

[27]

Fe3O4/SiO2/ MOF-177

MSPE-GC/MS

Phenols

Water

[28]

MOF-199/CNTs

HS-SPME-GC/MS

Ethylene, alcohols

Fruit

[20]

GO/MOF-199

HS-SPME-GC

Pesticides

Water, soil

[29]

Fe3O4/MOF-199

M-D-mSPEUHPLC

PAHs

Waters, fruit-tea

[30]

Fe3O4/MOF-199

MSPE-HPLC-MS/ MS

Insecticides

Water

[31]

Fe3O4/MOF-199

MSPE-AAS

Metal ions

Fish

[32]

Fe3O4/MOF-199

MSPE-AAS

Metal ions

Food

[33]

MOF-199/Fe3O4/ GA-MIP

UA-M-D-mSPEUV-Vis

Gallic acid

Urine, plasma, water

[34]

Grafted Fe3O4/ MOF-235

MSPE-AAS

Metal ions

Water

[35]

References

Table 10.1, MOF-5 [18,22e26], IRMOF-3 [27], MOF-177 [28], MOF-199 [20,29e34], and MOF-235 [35] have been applied in SPE. Magnetic SPE (MSPE) is the preferred extraction mode of IRMOF-based SPE. After extraction procedure, various analysis and detection methods, including gas chromatography (GC), liquid chromatography (LC), high-performance liquid chromatography (HPLC), and atomic absorption spectrum (AAS) can be selected according to target analytes and sample matrix.

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10.2.1

Solid-Phase Extraction

MOF-5-based sorbent for SPE

10.2.1.1 Directly used as sorbent substrate MOF-5 is the most famous MOF first reported by Yaghi’s group [1]. It can be used for gas extraction, separation, and storage. However, when used in sample preparation, its powder form is inconvenient. To address the problems in sample processing, Hu and coworkers proposed a facile method for immobilization of MOF-5 on porous copper support and its application in the noninvasive sampling of volatile organic sulfur compounds from plants [18]. The sorbent preparation and extraction procedures are illustrated in Fig. 10.2 and summarized below.

10.2.1.1.1

Preparation of Cu/MOF-5 bar

10.2.1.1.2

Headspace microextraction procedure

In situ solvothermal synthesis: foamed copper support was introduced into N,N0 dimethylformamide (DMF) solution containing Zn(NO3)2$6H2O and terephthalic acid. Crystal growth in an autoclave at 120 C for 24 h. Activation: under reduced pressure at 250 C for 24 h.

MOF-5 bar was exposed to the headspace of a hexane sample solution for 30 min.

Figure 10.2 The scheme of in situ solvothermal growth of Cu/MOF-5 bar for extraction of volatile sulfides from plants. This figure is reprinted with permission from Hu YL, Lian HX, Zhou LJ, Li GK. In situ solvothermal growth of metal-organic framework-5 supported on porous copper foam for noninvasive sampling of plant volatile sulfides. Anal Chem 2015;87(1):406e12. Copyright (2015) American Chemical Society.

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After extraction, MOF-5 bar was inserted into thermal desorption inlet immediately for thermal desorption.

10.2.1.1.3

Application

The Cu/MOF-5 bar is a robust enrichment medium benefiting both from the large surface area of MOFs and the three-dimensional rigid skeleton of microporous Cu network. This MOF-5-based sorbent is an excellent adsorbent for trapping volatile organic sulfur compounds from Chinese chive and garlic sprout and for the preconcentration of other plant volatiles.

10.2.1.2 MOF-5 composite materials for extraction The magnetic composite format is the preferred approach to enhance the stability and process capability of the IRMOF series materials for extraction from aqueous solutions. The chemical bonding between the magnetic nanoparticles and MOFs endows the composite material with higher chemical stability, durability, and improved reproducibility. Li’s group reported a facile and efficient strategy for fabrication of hybrid magnetic MOF-5 nanoparticles and their application in trace enrichment from complex samples [24]. The sorbent preparation and extraction procedures are illustrated in Fig. 10.3 and summarized below.

10.2.1.2.1

Preparation of Fe3O4/MOF-5 composite

Amino-functionalized Fe3O4 preparation: Fe3O4 nanoparticles were synthesized by a microwave-assisted method [36]. Tetraethylorthosilicate was added to the mixture of Fe3O4 nanoparticles in a mixture of ethanol and ammonia maintained at 40 C for 24 h. After drying, the activated nanoparticles were reacted with 3-aminopropyltriethoxysilane (APTES) with continuous agitation for 7 h at room temperature to form the amino-functionalized Fe3O4 nanoparticles. The aminofunctionalized Fe3O4 nanoparticles were introduced into a DMF solution of Zn(OAc)2$2H2O and terephthalic acid. The composite material was formed in an autoclave at 120 C for 10 h.

10.2.1.2.2

Extraction procedure

The Fe3O4/MOF-5 composite was dispersed into n-hexane sample solution with ultrasonication. After extraction, Fe3O4/MOF-5 composite was isolated with a magnet and the analytes eluted with acetonitrile containing 1% formic acid with ultrasound assistance. The supernatant was then evaporated under nitrogen for protection. Finally, the analytes were dissolved in a suitable solvent for analysis.

10.2.1.2.3

Application

The chemical bonding between the Fe3O4 and MOF-5 allows the hybrid material to be reused without measurable loss of performance and affords high enrichment. These advantages were demonstrated by the enrichment of trace PAHs in soil, seaweed, and fish tissue prior to GC/MS analysis and for gibberellic acids in plants prior to LC-MS/MS analysis.

290

Solid-Phase Extraction

(A)

APTES

TEOS Fe3O4

Fe3O4-OH

H2BDC

Zn2+

Fe3O4-NH2

MOF-5

magnetic MOF-5

(B)

Desorption

Enrichment

Analysis

Sample

Abundance(x105)

10 8 6 4 2 0 Magnetic separation

4 6 8 10 12 14 16 18 Time (min)

Figure 10.3 The scheme of Fe3O4/MOF-5 composite preparation (A) and sample analysis procedures (B). This figure is reprinted with permission from Hu YL, Huang ZL, Liao J, Li GK. Chemical bonding approach for fabrication of hybrid magnetic metal-organic framework-5: high efficient adsorbents for magnetic enrichment of trace analytes. Anal Chem 2013;85(14):6885e93 Copyright (2013) American Chemical Society.

10.2.2

MOF-199-based sorbent for SPE

MOF-199 is another typical IRMOF series material. Comparing with MOF-5, its higher porosity and larger pore size facilitate the adsorption of larger molecules. Its application for SPE remains limited by its moisture sensitivity. A simple and mild multilayer interparticle linking strategy was used by Li’s group for the preparation of a carbon nanotube (CNTs) hybrid MOF-199 [20]. With the assistance of the hydrophobic “shield” provided by the CNTs, MOF-199 was used for extraction under high humidity conditions. The sorbent preparation and extraction procedures are illustrated in Fig. 10.4 and summarized below.

10.2.2.1 Preparation of MOF-199/CNTs coated fibers A Cu(CH3COO)2 solution was added to the CNTs dispersed in DMF/EtOH containing 1,3,5-tricarboxybenzene and stirred for 8 h. The composite material was dried at 120 C overnight and sieved to a particle size less than 106 mm. Clean silica fibers

Metal-organic frameworks

291

Figure 10.4 The schematic working principle of MOF-199-based extraction with the protection of CNTs. This figure is reprinted with permission from Zhang ZM, Huang YC, Ding WW, Li GK. Multilayer interparticle linking hybrid MOF-199 for noninvasive enrichment and analysis of plant hormone ethylene. Anal Chem 2014;86(7):3533e3540. Copyright (2014) American Chemical Society.

were immersed into APTES for 3 min, dipped into the MOF-199/CNTs composite ethanol solution, and then dried at 70 C for 1 h.

10.2.2.2 Solid-phase microextraction procedure Weighed fruits were placed in a 500 mL sealed vial and purged with nitrogen. MOF199/CNTs coated fibers were then exposed to the vial headspace for 20 min at room temperature and transferred to GC inlet for analysis.

10.2.2.3 Application The multilayer interparticle linking strategy is a promising approach for the preparation of MOF-199/CNTs coating for extracting trace ethylene from fruits. Hybridization of hydrophobic materials will expand the application of MOFs to samples where moisture-resistance is important.

10.3

MIL in solid-phase extraction

Materials of Institute Lavoisier (MIL) series are an important type of MOFs material with high porosity and large surface areas. MIL series are constructed from various transition metals and dicarboxylic acid ligands, including succinic acid and glutaric

292

Solid-Phase Extraction

acid [37]. Moreover, trivalent metals such as chromium, vanadium, aluminum, and iron can be used for their synthesis in combination with terephthalic acid and tribenzoic acid [38]. Unlike the IRMOF series, the MIL series materials are stable in water, allowing their use in humid and aqueous environments. Hu and coworkers reported a MIL-101(Cr) packed column for the online analysis of naproxen and its metabolite in urine [39]. Through magnetization, the procedural steps for MIL-based SPE can be simplified. A magnetic MIL-100(Cr) composite was used for the analysis malachite green from water and fish samples by Maya and coworkers [40]. Furthermore, sensitivity, selectivity, and biological compatibility of MIL-based sorbent can be enhanced by adopting other modification approaches. As shown in Table 10.2, MIL-53 [41e45], MIL-68 [46], MIL-88 [47,48], MIL-96 [49], MIL-100 [40,50e57], and MIL-101 [39,58e85] have been used in SPE, and of these, MIL-101 is the most widely used. Magnetic solid-phase extraction (MSPE) is the preferred extraction format due to its easy operation. Some online strategies have been described to facilitate automation of sample processing [39,44,66].

10.3.1

MIL-100-based sorbent for SPE

MIL-100 is stable in water and strongly desired for extraction from aqueous samples. Moreover, the pore size of MIL-100 is relatively large and may be used to trap target compounds of high molecular weight. In addition, by magnetization, magnetic MIL-100-based SPE provides an easy operating mode for extraction. However, its hydrophobicity reduces sample interactions in aqueous solution. This drawback was solved by the automated method reported by Maya and coworkers [40]. The sorbent preparation and SPE procedures are summarized below:

10.3.1.1 Preparation of Fe3O4/MIL-100 composite Chromium trioxide, trimesic acid, and hydrofluoric acid were added to an autoclave and reacted at 220 C for 96 h. FeCl2$4H2O and FeCl3 were added to MIL-100(Cr) solution and stirred under nitrogen at room temperature for 2 h.

10.3.1.2 Automated magnetic dispersive micro-solid-phase extraction (automatic M-D-mSPE) extraction procedure The extraction procedure is illustrated in Fig. 10.5. Step 1, loading magnet and Fe3O4/MIL-100(Cr) sorbent. Step 2, loading sample solution. Step 3, extraction in the stop flow mode. Step 4, elimination of the sample matrix. Step 5, introducing eluent. Step 6, transfer eluent to a spectrophotometric detector.

10.3.1.3 Application This in-syringe M-D-mSPE strategy provides adequate dispersion and simplifies the extraction process using MIL-100(Cr) as a sorbent for extraction of malachite green from water and fish samples with direct spectrophotometric detection. Application of

Metal-organic frameworks

293

Table 10.2 Application of MIL in SPE. MOFs-based sorbent

Analysis methods

Analyte

Sample matrix

References

MIL-53(Al)

D-mSPE-UPLCMS/MS

Hormones

Water, urine

[41]

MIL-53(Al)

VA-D-mSPEHPLC

Phenols

Water

[42]

MIL-53(Al)polymer

SPE-HPLC

Drugs

Water, urine

[43]

MIL-53(Al)polymer

Online SPE-HPLC

Hormones

Urine

[44]

MIL-53(Fe)eC

MSPE-HPLC

EDCs

Food

[45]

PEEK/MIL-68(Al)

SBSE-HPLC-MS/ MS

Parabens

Cosmetics, plasma

[46]

MIL-88(Fe)

SPME-GC-MS

PCBs

Water, soil

[47]

MIL-88(Fe)/GO

SPME-GC

Plasticizers

Vegetable oil

[48]

MIL-96(Al)

SPME-GC-MS

Disinfection byproducts

Water

[49]

Fe3O4/MIL-100(Cr)

Automatic M-DmSPE-Vis

Dye

Water, fish

[40]

Fe3O4/MIL-100(Cr)

MSPE-GCeMS/ MS

PCBs

Water

[50]

MIL-100(Fe)eC

MSPE-GC

PAHs

Water

[51]

Ionic liquid-MIL100(Fe)

VA-D-mSPE-GC

PAHs

Water, food

[52]

Fe3O4/MIL-100(Fe)

MSPE-HPLC

Insect repellent

Water

[53]

Fe3O4/MIL-100(Fe)

MSPE-HPLC

Additive

Toothpastes

[54]

Fe3O4/MIL-100(Fe)

MSPE-LC-MS/ MS

Peptides

Serum

[55]

Fe3O4/MIL-100(Fe)/ GO

MSPEfluorescence

Drugs

Food

[56]

Grafted Fe3O4/MIL100(Fe)

M-D-mSPE-HPLC

Hormones

Water, urine

[57]

MIL-101(Cr)

SPE-DART-MS

Herbicides

Water

[58]

MIL-101(Cr)

SPE-UPLC-MS/ MS

Drugs

Water

[59] Continued

294

Solid-Phase Extraction

Table 10.2 Application of MIL in SPE.dcont’d MOFs-based sorbent

Analysis methods

Analyte

Sample matrix

References

MIL-101(Cr)

DSPE-GC-MS

EDCs

Water

[60]

MIL-101(Cr)

VA-D-SPEUPLC-MS/MS

Drugs

Plasma

[61]

MIL-101(Cr)

Online SPE-HPLC

Drugs

Urine

[39]

MIL-101(Cr)polymer

SPME-CEC

Penicillin

Water

[62]

MIL-101(Cr)/PVA

VA-D-SPEHPLC-MS/MS

Drugs

Water

[63]

MIL-101(Cr)/RGO

SPE-HPLC/ MALDI-TOFMS

Drugs, proteins

Water

[64]

PDMS/MIL-101(Cr)

SBSE-GC

Pesticides

Water

[65]

TiO2/MIL-101(Cr)

Online SPE-GCMS

Volatile pollutants

Air

[66]

Fe3O4/MIL-101(Cr)

MSPE-GC

Pesticides

Water, tea

[67]

Fe3O4/MIL-101(Cr)

MSPE-GC

Pesticides

Water

[68]

Fe3O4/MIL-101(Cr)

M-D-SPE-GC-MS

Plasticizers

Water, plasma

[69]

Fe3O4/MIL-101(Cr)

MSPE-HPLC

Hormones

Water

[70]

Fe3O4/MIL-101(Cr)

MSPE-HPLC

Dyes

Food

[71]

Fe3O4/MIL-101(Cr)

MSPE-UPLC-MS/ MS

Drugs

Water

[72]

Fe3O4/MIL-101(Cr)

MSPE-MALDITOF-MS

Proteins

Cell

[73]

Grafted Fe3O4/ MIL-101(Cr)

MSPE-IC

Azide

Sartan drugs

[74]

Grafted Fe3O4/ MIL-101(Cr)

MSPE-AAS

Metal ions

Water

[75]

Grafted Fe3O4/ MIL-101(Cr)

MSPE-AAS

Metal ions

Water, food

[76]

Fe3O4/SiO2/ MIL-101(Cr)

MSPE-HPLC

PHAs

Water

[77]

Fe3O4/SiO2/ MIL-101(Cr)

MSPE-HPLC

Pesticides

Water

[78]

Fe3O4/SiO2-GO/ MIL-101(Cr)

MSPE-HPLC

Herbicides

Food

[79]

Metal-organic frameworks

295

Table 10.2 Application of MIL in SPE.dcont’d MOFs-based sorbent

Analysis methods

Analyte

Sample matrix

References

Fe3O4/MIL-101(Fe)

MSPE-GC

Pesticides

Hair, urine

[80]

Fe3O4/MIL-101(Fe)

MSPE-AAS

Heavy metal ions

Plants

[81]

Fe3O4/MIL-101(Fe)

MSPE-AAS

Metal ions

Seafood

[82]

Fe3O4/MIL-101(Fe)

MSPE-AAS

Metal ions

Fish

[83]

Fe3O4/MIL-101(Fe)

MSPE-HPLC

Fungicides

Water

[84]

Fe3O4/MIL-101(Fe)

MSPE-HPLC

Herbicides

Water, vegetable

[85]

Figure 10.5 The scheme of automatic M-D-mSPE procedure. This figure is reprinted with permission from Maya F, Palomino Cabello C, Estela JM, Cerda V, Palomino TG. Automatic in-syringe dispersive micro-solid phase extraction using magnetic metal-organic frameworks. Anal Chem 2015;87(15):7545e7549. Copyright (2015) American Chemical Society.

this method can be extended to other water stable MOFs, as well as other target analytes with suitable detectors.

10.3.2 MIL-101-based sorbent for SPE MIL-101 has an even larger pore size than MIL-100 with retention of its water/solvent stability. MIL-101(Cr) was used by Li’s group for the extraction of naproxen [39]. Fig. 10.6 demonstrates that MI-101(Cr) is a more efficient sorbent for the extraction of naproxen than C18-bonded silica and multiwalled nanotubes (MWNTs). The

296

Solid-Phase Extraction

Figure 10.6 Adsorption isotherms for naproxen on different SPE sorbents at 30 C. This figure is reprinted with permission from Hu YL, Song CY, Liao J, Huang ZL, Li GK. Water stable metal-organic framework packed microcolumn for online sorptive extraction and direct analysis of naproxen and its metabolite from urine sample. J Chromatogr A 2013;1294:17e24 Copyright (2013) Elsevier.

preparation of the sorbent microcolumn for online extraction by micro-solid-phase extraction coupled to liquid chromatography (mSPE-HPLC) is shown in Fig. 10.7.

10.3.2.1 Preparation of MIL-101(Cr)-based microcolumn To prepare the MIL-101(Cr) Cr(NO3)3$9H2O, terephthalic acid, and hydrofluoric acid were mixed and heated in an autoclave at 220 C for 8 h. The MOF was activated by heating at 150 C for 24 h under reduced pressure. The MIL-101(Cr) was dispersed in the slurry with ultrasonic ethanol assistance and immediately pumped into a polyether ether ketone (PEEK) tube.

10.3.2.2 The online micro-solid-phase extraction procedure The extraction procedure is illustrated in Fig. 10.7. Step 1 preconditioning of the sorbent cartridge. Step 2, loading the sample solution. Step 3, desorption of naproxen from the microcolumn to the separation column. Step 4, flow rate adjustment for chromatographic separation.

10.3.2.3 Applications This MIL-101(Cr)-based online mSPE-HPLC system opens up a new strategy for the application of MOFs-based material as sorbents for analysis of polar compounds from an aqueous matrix.

Metal-organic frameworks

297

Figure 10.7 The scheme of fabrication of the MIL-101(Cr) packed microcolumn and the online mSPE-HPLC system [43]. This figure is reprinted with permission from Hu YL, Song CY, Liao J, Huang ZL, Li GK. Water stable metal-organic framework packed microcolumn for online sorptive extraction and direct analysis of naproxen and its metabolite from urine sample. J Chromatogr A 2013;1294:17e24 Copyright (2013) Elsevier.

10.4

ZIF in solid-phase extraction

Zeolitic imidazolate framework (ZIF), are water and solvent stable MOFs, synthesized from Zn(II) or Co(II) and imidazole ligands [86]. Their zeolite-like tetrahedron structure and chemical stability in boiling alkaline solution and organic solvents distinguish these materials [87]. These features facilitate their chemical modification with a wide range of reagents. Thus, the ZIF series materials have been widely exploited for SPE [88e97]. Yan’s group reported several applications of ZIF series materials, as well as modified ZIF-based sorbent for SPE [89,91,96]. Applications of ZIF-based SPE are summarized in Table 10.3.

10.4.1 ZIF-8-based sorbent for SPE Chang and coworkers used ZIF-8-based sorbent and molecular sieves to for the analysis of n-alkanes from human serum and fuel samples, Fig. 10.8 [91]. The extraction procedure is summarized below.

298

Solid-Phase Extraction

Table 10.3 Application of ZIF in SPE. MOFs-based sorbent

Analysis methods

Analyte

Sample matrix

References

ZIF-7, ZIF-11

SPE-HPLC

PAHs

Water

[88]

ZIF-8

Online SPE-HPLC

Drugs

Water, milk

[89]

ZIF-8

mSPE-GC-MS

PAHs

Water

[90]

ZIF-8

SPME-GC-MS

Alkanes

Fuel, serum

[91]

Cellulose/ZIF-8

SPE-HPLC

PAHs

Water

[92]

Fe3O4/ZIF-8

MSPE-AFS

Metal

Water, urine

[93]

ZIF-67-C

M-D-SPE-HPLC

Insecticides

Water, vegetable

[94]

ZIF-67-C

MSPE-HPLC

Herbicides

Plants

[95]

ZIF-90

SPME-GC

Endocrine disruptors

Water, soil

[96]

Fe3O4/ZIF-90/ trypsin

MSPE-MALDITOF-MS

Proteins, peptides

Cell

[97]

Figure 10.8 The scheme of ZIF-8-based tandem molecular sieves for trace analytes in a complex sample. This figure is reprinted with permission from Chang N, Gu ZY, Wang HF, Yan XP, Metalorganic-framework-based tandem molecular sieves as a dual platform for selective microextraction and high-resolution gas chromatographic separation of n-alkanes in complex matrixes. Anal Chem 2011;83(18):7094e101. Copyright (2011) American Chemical Society.

10.4.1.1 Preparation of ZIF-8 coated SPME fibers Zn(NO3)2$6H2O was added to 2-methylimidazole in methanol and mixed at room temperature for 1h. Clean stainless steel wires were dipped into a ZIF-8 DMF solution for 20 s, and dried in a nitrogen atmosphere at 250 C for 10 min in the GC injector. Ten coating cycles were applied to achieve the final SPME fiber.

Metal-organic frameworks

299

10.4.1.2 Headspace solid-phase microextraction procedure Fuel or human serum samples were introduced into a 100 mL sealed vial and sonicated at 25 C for 5 min. The ZIF-8 coated fiber was exposed to the vial headspace for 20 min and transferred to the GC inlet for analysis.

10.4.1.3 Application By coupling a ZIF-8-based sorbent for SPME with ZIF-8-based stationary phases for GC separation suitable enrichment and selectivity for the analysis of n-alkanes in human serum and fuel samples was achieved. This strategy can be adopted for other applications of MOFs materials.

10.5

UiO in solid-phase extraction

University of Oslo (UiO) series are zirconium-based novel MOFs materials [98]. Due to their outstanding physicochemical stability, the UiO has attracted considerable attention as an extraction sorbent [99e103]. Xia and coworkers used UiO-66 for the extraction of herbicides from vegetables [99]. Moreover, after suitable modification, UiO-66-based sorbent has been used for proteome research [101,102]. As shown in Table 10.4, functionalized UiO series materials are useful sorbents for the extraction of target compounds from biological samples.

10.5.1 UiO-66-based sorbent for SPE Zhou and coworkers synthesized titanium immobilized magnetic UiO-66 material for the extraction of low phosphopeptides from human serum and milk samples [101]. The UiO-66-based sorbent is not only suitable for metal-oxide affinity chromatography (MOAC) for the extraction of monophosphorylated peptides but also as an immobilized metal affinity chromatography (IMAC) material for polyphosphorylated peptides. Beyond enhanced sensitivity and selectivity, sample pretreatment is simplified by UiO-66-based sorbents, as summarized below and illustrated in Fig. 10.9.

10.5.1.1 Preparation of UiO-66-based sorbent Fe3O4 nanoparticles were synthesized by a hydrothermal reaction and then dispersed into dopamine hydrochloride to form a Fe3O4 and polydopamine (Fe3O4/PDA) composite material. Fe3O4/PDA composite was added to a DMF solution containing ZrCl4 and 2-aminoterephthalic acid and mixed at 120 C for 2 h. Three reaction cycles were used for the synthesis. The activated Fe3O4/UiO-66 composite was dispersed in a solution of adenosine triphosphate (ATP) and vibrated for 2 h. Ti(SO4)2 was then added to the ATP grafted Fe3O4/UiO-66 composite solution to immobilize the titanium (IV) ions.

300

Solid-Phase Extraction

Table 10.4 Application of UiO in SPE. MOFs-based sorbent

Analysis methods

Analyte

Sample matrix

References

UiO-66

D-mSPE- HPLC

Herbicides

Vegetable

[99]

Grafted Fe3O4/UiO-66

M-D-SPE-GC-MS

PCBs

Soil

[100]

Grafted Fe3O4/UiO-66

MSPE-MALDITOF-MS

Proteins, peptides

Serum, milk

[101]

Grafted Fe3O4/UiO66/Au

MSPE-MALDITOF-MS

Peptides

Serum

[102]

Fe3O4/SiO2/UiO-66

MSPE-HPLC-MS/ MS

Neurotoxin

Shellfish

[103]

Figure 10.9 The scheme of UiO-66-based dual-functionalized sorbent for analysis trace phosphorylated peptides from complex sample. This figure is reprinted with permission from Zhou JQ, Liang YL, He XW, Chen LX, Zhang YK. Dual-functionalized magnetic metal-organic framework for highly specific enrichment of phosphopeptides. Chem Eng 2017;5(12):11413e21. Copyright (2017) American Chemical Society.

10.5.1.2 Magnetic solid-phase extraction The UiO-66-based sorbent was added to a solution of human serum and incubated at room temperature for 30 min. After extraction, the UiO-66-based sorbent was isolated using a magnet and phosphopeptides eluted with 5% ammonia solution by shaking for 15 min.

Metal-organic frameworks

301

10.5.1.3 Application After modification, UiO-66-based sorbents exhibit several advantages, including biological compatibility and simple sample processing procedures [104].

10.6

Conclusions

MOFs are uniform structured porous materials suitable for solid-phase extraction. The application of MOFs materials for SPE needs to consider a wide range of sample properties to achieve the desired extraction performance. The water stability of MOFs-based sorbent is the first concern. Some MOFs can be used directly with aqueous samples, such as the MIL series, ZIF series and UiO series but not the IRMOF series sorbents. Postsynthesis modification with hydrophobic groups, composite materials with water-repellent components and carbonization are alternative approaches to improve water stability of moisture-sensitive MOFs. Beyond water stability, the selectivity of MOFs-based sorbents for the extraction of target analytes is another concern. The adsorption of analytes by many MOFs affords only low selectivity. To enhance the specific extraction of target compounds postsynthesis modifications enable MOFs-based sorbent to be modified for SPE. Packing monolithic columns with MOFs for mSPE and preparing magnetic MOFs composites for MSPE are the favored approaches to simplify processing procedures and for automation of sample processing.

List of abbreviations AAS AFS APTES ATP CNTs DMF EDCs DFMMOF DSPE GAs GC HPLC IMAC IC IRMOF LC mSPE M-D-mSPE MIL

Atomic absorption spectrum Atomic fluorescence spectrometry 3-aminopropyl triethoxysilane Adenosine triphosphate Carbon nanotubes N, N0 - dimethylformamide endocrine disrupting compounds Dual-functionalized magnetic metal-organic framework Dispersive solid-phase extraction Gibberellic acid Gas chromatography High-performance liquid chromatography Immobilized metal affinity chromatography Ion chromatography Isoreticular metal-organic framework Liquid chromatography Micro-solid-phase extraction Magnetic dispersive micro-solid-phase extraction Materials of Institute Lavoisier

302

Solid-Phase Extraction

MOAC Metal-oxide affinity chromatography MOFs Metal-organic frameworks MALDI-TOF MS Matrix-assisted laser desorption ionization time-of-flight mass spectrometry MSPE Magnetic solid-phase extraction PCBs Polychlorinated biphenyls MWNTs Multiwalled nanotube PEEK Polyetheretherketone PHAs Polycyclic aromatic hydrocarbons SBSE Btir bar sorptive extraction SPME Solid-phase microextraction UPLC Ultra-performance liquid chromatography UiO University of Oslo ZIF Zeolitic imidazolate framework

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[56] Li ZQ, Qi MY, Tu CY, Wang WP, Chen JR, Wang AJ. Magnetic metal-organic framework/graphene oxide-based solid-phase extraction combined with spectrofluorimetry for the determination of enrofloxacin in milk sample. Food Anal Methods 2017;10(12):4094e103. [57] Khezeli T, Daneshfar A. Dispersive micro-solid-phase extraction of dopamine, epinephrine and norepinephrine from biological samples based on green deep eutectic solvents and Fe3O4@MIL-100(Fe) core-shell nanoparticles grafted with pyrocatechol. RSC Adv 2015;5(80):65264e73. [58] Li XJ, Xing JW, Chang CL, Wang X, Bai Y, Yan XP, Liu HW. Solid-phase extraction with the metal-organic framework MIL-101(Cr) combined with direct analysis in real time mass spectrometry for the fast analysis of triazine herbicides. J Sep Sci 2014;37(12): 1489e95. [59] Dai XP, Jia XN, Zhao P, Wang T, Wang J, Huang PT, He L, Hou XH. A combined experimental/computational study on metal-organic framework MIL-101(Cr) as a SPE sorbent for the determination of sulphonamides in environmental water samples coupling with UPLC-MS/MS. Talanta 2016;154:581e8. [60] Huang ZZ, Lee JK. Performance of metal-organic frame work MIL-101 after surfactant modification in the extraction of endocrine disrupting chemicals from environmental water samples. Talanta 2015;143:366e73. [61] Qi C, Cai QQ, Zhao P, Jia XN, Lu N, He L, Hou XH. The metal-organic framework MIL101(Cr) as efficient adsorbent in a vortex-assisted dispersive solid-phase extraction of imatinib mesylate in rat plasma coupled with ultra-performance liquid chromatography/ mass spectrometry: application to a pharmacokinetic study. J Chromatogr A 2016;1449: 30e8. [62] Lin CL, Lirio S, Chen YT, Lin CH, Huang HY. A novel hybrid metal-organic frameworkpolymeric monolith for solid-phase microextraction. Chem Eur J 2014;20(12):3317e21. [63] Wang YD, Zhang Y, Cui JN, Li S, Yuan M, Wang T, Hu Q, Hou XH. Fabrication and characterization of metal organic frameworks/polyvinyl alcohol cryogel and their application in extraction of non-steroidal anti-inflammatory drugs in water samples. Anal Chim Acta 2018;1022:45e52. [64] Zhang XQ, Liang QL, Han Q, Wan W, Ding MY. Metal-organic frameworks@graphene hybrid aerogels for solid-phase extraction of non-steroidal anti-inflammatory drugs and selective enrichment of proteins. Analyst 2016;141(13):4219e26. [65] Xiao ZW, He M, Chen BB, Hu B. Polydimethylsiloxane/metal-organic frameworks coated stir bar sorptive extraction coupled to gas chromatography-flame photometric detection for the determination of organophosphorus pesticides in environmental water samples. Talanta 2016;156e157:126e33. [66] Hu YL, Huang ZL, Zhou LJ, Wang DM, Li GK. Synthesis of nanoscale titania embedded in MIL-101 for the adsorption and degradation of volatile pollutants with thermal desorption gas chromatography and mass spectrometry detection. J Sep Sci 2014;37(12): 1482e8. [67] Lu N, He X, Wang T, Liu SY, Hou XH. Magnetic solid-phase extraction using MIL101(Cr)-based composite combined with dispersive liquid-liquid microextraction based on solidification of a floating organic droplet for the determination of pyrethroids in environmental water and tea samples. Microchem J 2018;137:449e55. [68] He X, Yang W, Li SJ, Liu Y, Hu BC, Wang T, Hou XH. An amino-functionalized magnetic framework composite of type Fe3O4-NH2@MIL-101(Cr) for extraction of pyrethroids coupled with GC-ECD. Microchim Acta 2018;185(2):125e33. [69] Dargahi R, Ebrahimzadeh H, Asgharinezhad AA, Hashemzadeh A, Amini MM. Dispersive magnetic solid-phase extraction of phthalate esters from water samples and

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human plasma based on a nanosorbent composed of MIL-101(Cr) metal-organic framework and magnetite nanoparticles before their determination by GC-MS. J Sep Sci 2018;41(4):948e57. Huang YF, Liu M, Wang YQ, Li Y, Zhang JM, Huo SH. Hydrothermal synthesis of functionalized magnetic MIL-101 for magnetic enrichment of estrogens in environmental water samples. RSC Adv 2013;6(19):15362e9. Shi XR, Chen XL, Hao YL, Li L, Xu HJ, Wang MM. Magnetic metal-organic frameworks for fast and efficient solid-phase extraction of six Sudan dyes in tomato sauce. J Chromatogr B 2018;1086:146e52. Wang T, Liu SY, Gao GH, Zhao P, Lu N, Lun XW, Hou XH. Magnetic solid phase extraction of non-steroidal anti-inflammatory drugs from water samples using a metal organic framework of type Fe3O4/MIL-101(Cr), and their quantitation by UPLC-MS/MS. Microchim Acta 2017;184(8):2981e90. Wei JP, Qiao B, Song WJ, Chen T, li F, Li BZ, Wang J, Han Y, Huang YF, Zhou ZJ. Synthesis of magnetic framework composites for the discrimination of Escherichia coli at the strain level. Anal Chim Acta 2015;868:36e44. Zhang SN, Han PP, Xia Y. þFacile extraction of azide in sartan drugs using magnetized anion-exchange metal-organic frameworks prior to ion chromatography. J Chromatogr A 2017;1514:29e35. Rezabeyk S, Manoochehri M. Speciation analysis of Tl(I) and Tl (III) after magnetic solid phase extraction using a magnetite nanoparticle composite modified with aminodibenzo18-crown-6 functionalized MIL-101(Cr). Microchim Acta 2018;185(8):365e73. Kalantari H, Manoochehri M. A nanocomposite consisting of MIL-101(Cr) and functionalized magnetite nanoparticles for extraction and determination of selenium (IV) and selenium (VI). Microchim Acta 2018;185(3):196e204. Huo SH, Yan XP. Facile magnetization of metaleorganic framework MIL-101 for magnetic solid-phase extraction of polycyclic aromatic hydrocarbons in environmental water samples. Analyst 2012;137(15):3445e51. Ma JP, Yao ZD, Hou LW, Lu WH, Yang QP, Li JH, Chen LX. Metal organic frameworks (MOFs) for magnetic solid-phase extraction of pyrazole/pyrrole pesticides in environmental water samples followed by HPLC-DAD determination. Talanta 2016;161: 686e92. Liang L, Wang XH, Sun Y, Ma PY, Li XP, Piao HL, Jiang YX, Song DQ. Magnetic solidphase extraction of triazine herbicides from rice using metal-organic framework MIL101(Cr) functionalized magnetic particles. Talanta 2018;179:512e9. Zhang SL, Jiao Z, Yao WX. A simple solvothermal process for fabrication of a metalorganic framework with an iron oxide enclosure for the determination of organophosphorus pesticides in biological samples. J Chromatogr A 2014;1371:74e81. Babazadeh M, Khanmiria RH, Abolhasania J, Kalhora EG, Hassanpour A. Solid phase extraction of heavy metal ions from agricultural samples with the aid of a novel functionalized magnetic metal-organic framework. RSC Adv 2015;5(26):19884e92. Ghorbani-Kalhor E, Hosseinzadeh-Khanmiri R, Abolhasani J, Babazadeh M, Hassanpour A. Determination of mercury (II) ions in seafood samples after extraction and preconcentration by a novel functionalized magnetic metal-organic framework nanocomposite. J Sep Sci 2015;38(7):1179e86. Babazadeh M, Khanmiri RH, Abolhasani J, Ghorbani-Kalhor E, Hassanpour A. Synthesis and application of a novel functionalized magnetic metal-organic framework sorbent for determination of heavy metal ions in fish samples. Bull Chem Soc Jpn 2015;88(6):871e9.

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Electrospun nanofibers Habib Bagheri, Omid Rezvani, Shakiba Zeinali, Sara Asgari, Tahereh Golzari Aqda, Faranak Manshaei Environmental and Bio-Analytical Laboratories, Department of Chemistry, Sharif University of Technology, Tehran, Iran

11.1

11

Introduction

There has been remarkable progress in material science and the development of new types of natural and synthetic structures with different properties over the last few decades. These materials have evolved from structures based on small molecules to macromolecules and polymers. Today, emphasis is placed on materials at the nanoscale [1]. A more generalized description of nanotechnology was established by the national nanotechnology initiative, which defines nanotechnology as the manipulation of matter with at least one dimension sized from 1 to 100 nm. This definition reflects the fact that quantum mechanical effects are more significant at this quantum-realm scale. Two main approaches are used in nanotechnology. In the “bottom-up” approach, materials and devices are built from molecular components which assemble themselves chemically by principles of molecular recognition [2]. In the “top-down” approach, nanoobjects are constructed from larger entities without atomic-level control [3]. The ever-growing library of nanostructures with variable sizes, shapes, and compositions has opened up many opportunities for the design of functional materials [4]. Controlling material structure at the molecular level has been enabled by innovative nanoscale manipulations of surface area, surface functionality, and porosity [5]. These developments are responsible for several enhancements in the field of separation and filtration [6]. Thus, these profound investigations have contributed to the synthesis and advancement of various forms of nanomaterials such as nanofibers, nanoparticles, nanotubes, and nanowires. Nanofibers, owing to their large surface area, high aspect ratio, flexibility, and other desirable features have garnered significant interests from both academia and industry. Several methods have been utilized for producing nanofibers, including electrospinning, melt or solution blowing, phase separation, self-assembly, and template synthesis. Among them, electrospinning is the most versatile technique for the synthesis of nanofibers from different materials, that is, polymers, ceramics, and metals. It is interesting that the history of the electrospinning process goes back to the observation of water behavior under the influence of an electric field [7]. Cooley [8] and Morton [9] were the first to describe electrospinning for fiber production.

Solid-Phase Extraction. https://doi.org/10.1016/B978-0-12-816906-3.00011-X Copyright © 2020 Elsevier Inc. All rights reserved.

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Solid-Phase Extraction

Electrospinning process

In electrospinning, a rather high voltage is applied to a polymeric solution in order to induce electrical charges within the fluid. When these charges reach a threshold value, a fluid jet will eject from the droplet at the tip of a needle resulting in the formation of a Taylor cone [10]. The produced jet dries during its flight toward a collecting electrode while simultaneously elongated by a whipping process caused by electrostatic repulsion at small bends in the fibers [11]. The elongation and thinning of the fibers resulting from this bending instability lead to the formation of uniform fibers with submicrometer-scale diameters [12]. Modification of the spinneret or the type of polymeric solution can generate miscellaneous fiber-fabricating protocols with unique structures and properties [13e16]. Electrospun fibers can adopt a porous or core-shell morphology depending on the type of materials being spun as well as the evaporation rates and solvent miscibility. For multiple spinning fluids, the general criteria for the creation of fibers, depends upon the spinnability of the outer solution. Accordingly, electrospinning is performed in three different formats: coaxial, emulsion, and melt electrospinning. A coaxial setup for a dual-solution system, allowing the inner feed to meet the outer one at the tip of the spinneret is shown in Fig. 11.1 [17,18]. If the solutions are immiscible then a core-shell structure is usually observed. Emulsions can also be used to create core-shell or composite fibers without modification of the spinneret [14]. However, these fibers are usually more difficult to produce compared with coaxial spinning due to the greater number of variables which need to be taken into account. Electrospinning of polymer melts eliminates the need for volatile solvents typically used in solution electrospinning. This strategy allows semicrystalline polymer fibers such as PE, PET, and PP to be conveniently fabricated, which seem otherwise impossible or very difficult using solution spinning [19]. The setup is very similar to that employed for conventional electrospinning and includes a syringe or spinneret, a high voltage supply, and a collector. The polymer melt is usually produced by either resistance heating, circulating fluids, air heating, or lasers. However, electrospinning is

Figure 11.1 A coaxial electrospinning nozzle for fabricating core-shell nanofibers [18].

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usually carried out using a polymeric solution. Similar to most fabricating processes, there are many parameters which influence the morphology of the electrospun fibers, for example, beaded or porous fibers. Generally, the influential parameters for conventional electrospinning are associated with the applied voltage, temperature, collector, and ambient conditions [20,21]. With the understanding of these parameters, it is possible to devise setups for fibrous structures of various forms and arrangements. It is also possible to create nanofibers with different morphologies as the influential parameters are varied.

11.2.1 Polymer solution The properties of the polymer solution including the molecular weight [20], solution viscosity [22], surface tension [22], solution conductivity [23], and dielectric effect of solvent have the most significant influence on the electrospinning process and the resultant fibers’ morphology. The surface tension is particularly influential for the formation of beads along the fiber length. The viscosity of the solution and its electrical properties determine the extent of elongation. This will in turn have an effect on the diameter of the electrospun fibers. Thus if the conductivity of the solution is increased, more charges can be carried by the electrospinning jet. The conductivity of the solution can be enhanced by the addition of ions. As previously mentioned, bead formation will occur if the solution is not fully stretched. Therefore, when a small amount of salt or polyelectrolyte is added to the solution, the increased charges carried by the solution will increase the stretching of the solution.

11.2.2 Processing conditions The electrospinning jet is influenced by various processing factors, including the applied voltage [24], the feed-rate [25], temperature of the solution, type of collector, diameter of the needle, and distance between the needle tip and collector [26]. These parameters are quite effective for altering the fiber morphology. The high voltage will firstly induce the electrical charges on the solution at the tip of the needle, creating electrostatic coulombic forces on the surface of the Taylor Cone. As a result, a competitive condition between the repulsion forces and surface tension is generated leading to a critical point under which electrostatic forces overcome the solution surface tension. Generally, both high negative or positive voltages (>6 kV) is necessary for fabricating fibers deposited at the collector. The solution feed rate is a key element of the electrospinning process. When the feed rate is raised, there is an increase in fiber diameters as well as the number of beads. The solution temperature not only helps to facilitate the evaporation of the solvent, but also reduces the solution viscosity of the polymer solution. For instance, when polyurethane is electrospun at a higher temperature, the fibers have a more uniform diameter. This may be due to the lower viscosity of the solution and greater solubility of the polymer in the solvent which allows more stretched fibers to be formed. The internal diameter of the needle or the pipette orifice also has a characteristic effect on the electrospinning process. By selecting a needle with a smaller internal diameter,

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the clogging as well as the amount of beads within the fiber structure are reduced. The decrease in the internal diameter of the orifice leads to the production of electrospun fibers with reduced diameters.

11.2.3

Ambient parameters

The effect of the surrounding area of the electrospinning jet is still poorly understood. Any interaction between the surrounding and the polymeric solution may have an effect on the electrospun fiber morphology. High humidity [27], for example, was found to cause the formation of pores on the surface of the fibers. Since electrospinning is influenced by external electric fields, any change in its environment will also affect the formation process. At high humidity, it is likely that water condenses on the surface of the fibers. As a result, this may have an influence on the fiber morphology, especially for polymers which are dissolved in volatile solvents. The air composition is also influential in the electrospinning process. Different gases behave differently in high electrostatic fields. For example, helium will break down in high electrostatic fields making fiber fabrication more laborious. Generally, the surrounding pressure affects the resulting fibers. When the pressure is below atmospheric pressure, the polymer solution in the syringe will have a greater tendency to flow out of the needle causing unstable jet initiation.

11.3

Characterization of electrospun nanofibers

Solid phase extraction is a surface process, in which analytes are retained on the surface of extractive media. It means that each surface feature can influence the extraction efficiency. After preparation of electrospun sorbents, their aspects should be characterized. As explained earlier, some chemical, instrumental, and ambient parameters influence the final shape of submicron fibers, including porosity, surface area, fiber shape, and continuity. In addition, some of the instrumental and ambient parameters are uncontrollable or difficult to control. Consequently, the extraction performance of the fibers depends on their morphologies and chemical structures which are mostly unpredictable [28]. The fibers are often characterized on the basis of physical and chemical methods.

11.3.1

Physical fiber parameters

There are several surface relevant parameters including fiber diameters and their size distribution, porosity and surface roughness, which greatly affect extraction properties.

11.3.1.1 Fiber diameter and size distribution The simplest method for evaluating fiber morphology is light microscopy. Although, the method needs no sample preparation and is relatively straightforward, resolution is low and magnification is limited. Other imaging methods, including scanning electron

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microscopy (SEM) and transmission electron microscopy (TEM), are based on the use of an electron beam as incident source. In SEM devices an electron beam is projected onto the sample surface and the reflected electrons are collected. The most common collected signals are secondary electrons (SE) from excited atoms. The number of these electrons depends on the surface morphology. The collection of these electrons by the detector provides a morphological image which is quite useful to screen the surface. The alternative signal is from back-scattered electrons (BSE) reflected from the surface and depends on the atomic number. Thus, heavier atoms reflect more electrons toward the collecting electrode located above the surface. The more heavy atoms present in the fiber structure, the brighter the final image. This effect is mostly applied to imaging metal composite fibers to reveal the distribution of metallic particles throughout the fiber structure. Conventional SEM systems use thermoionic guns as an electron source, which requires heating an electron source such as tungsten for electron production. For these systems low brightness and thermal drifting during imaging are the main problems. To address this issue, a new generation of SEM instruments was introduced utilizing an electrical potential gradient source to overcome the work function and facilitate electrons release. This system is called field emission SEM (FE-SEM) and produces images with higher resolution. For instance, Ma et al. prepared a core-shell poly(vinylpyrrolidone) (PVP)-Ag NP electrospun fibers in which PVP acted as a reducing agent for production of Ag NPs from AgNO3 during electrospinning. This concept was confirmed by observing the SEM images (Fig. 11.2A) [29]. It is also possible to calculate the fiber diameter distribution using SEM images [31]. There are two general pathways in this regard. The first method is to take several images from different parts of the fibers, and subsequently calculate fiber diameters with the aid of SEM software or Photoshop. A histogram of frequency versus fiber diameters is finally established. The drawback of this method is the bias of the results by inappropriate sampling. The other method calculates a fiber diameter distribution by the use of software, such as ImageJ. Coutinho et al. synthesized cellulose nanofibers in ionic liquid at room temperature. They used SEM images (Fig. 11.2B) to calculate fiber diameters and demonstrated the production of nanosized fibers [30]. This was a difficult task using other methods.

(A)

(B)

Figure 11.2 SEM images (A) of deposited Ag NPs on electrospun PVP surface [29] (B) used for the fiber diameter measurement [30].

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Solid-Phase Extraction

Figure 11.3 (A) TEM image recorded from the core-shell polyethylene oxide/polylactic acid [32] and (B) hollow TiO2/PVP [33].

TEM imaging is analogous to SEM in that both use electron beam. However, in TEM instruments the electron beam is transmitted from the sample being and collected by a detector located beneath the sample stage at the opposite side of electrons beam pathway. TEM images are particularly useful for imaging the electrospun hollow and core-shell fibers [31]. Xu et al. fabricated a core/shell polyethylene oxide/poly(lactic acid) structure. The TEM images (Fig. 11.3A) confirmed the core/shell structure and diameter of the fibers [32]. Also, Xia et al. prepared electrospun TiO2/PVP hollow fibers. The core consisted of heavy mineral oil which was subsequently removed by immersion in isooctane. The surface smoothness and hollow structure of the fibers were characterized by TEM (Fig. 11.3B) [33].

11.3.1.2 Porosity and topography Porosity is a measure of the void (i.e., “empty”) spaces in a material, and is considered as the fraction of void volumes compared with the total volume. Since in solid phase extraction techniques, the analytes are mostly adsorbed by the surface of the extracting phase, the porosity or surface area is of great importance. As mentioned earlier, higher surface porosity leads to higher surface areas and better extraction efficiency. There are different methods for calculating surface porosity. The conventional method is mercury porosimetry, which pushes mercury into holes and pores of the material by pressure. Since mercury does not usually wet the surface, it is the only candidate to be applied in this method. Finally the applied pressure and volume of mercury are converted to surface porosity with mathematical calculations. This method is not common because of experimental difficulties and the toxicity of mercury [34]. The alternative is the BrunauereEmmetteTeller (BET) method. This method is based on the adsorption of gases on a solid surface. Gases should be inert to avoid chemical interaction with the sample. An equation correlates gas pressure to the amount of adsorbed gas from which the specific surface area is obtained [35]. Xia et al. described a new method of electrospinning aimed at increasing the specific surface area of the fibers. They used a liquid nitrogen bath before the collector to induce thermal phase separation. The surface areas of the resulting fibers were

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threefold higher than theoretical values [36]. Yang et al. prepared porous carbon nanofibers with two different polymer mixtures. The electrospun nanofibers were thermally treated by which one of the polymers was removed and other was carbonized to achieve hollow and highly porous carbon fibers. Different ratios of the two polymers provided sufficient porosity with surface areas from 800 to 940 m2 g
1. These data were obtained by the BET technique [37]. Surface topology is another important characteristic of solid extraction phases which provides information on surface smoothness and roughness. Scanning probe microscopy produces a 3D image of surface topology. The atomic force microscope (AFM) is the most common technique, since it is applicable to a wide variety of materials with no need for sample preparation. The interaction between the needle tip and surface atoms leads to an image from which the smoothness/roughness of the surface can be predicted. The main advantage of this technique is the fact that the surface morphology remains unchanged during imaging. Xu et al. provided an AFM map and surface topography of polyethylene oxide/chitosan to illustrate the structural evolution of the nanofibers during electrospinning. They demonstrated that fiber formation is not just a simple drying and thinning process but consists of some surface topologically different steps, including columnar, platelike, bumpy, and other surface features (Fig. 11.4) [38]. There are other less common methods for characterizing the mechanical properties of fibers such as the vibrating sample magnetometer for magnetic properties [39], air permeability testing [40], conductivity measurement [41], and tensile testing [42]. However these properties are not of great significance for the use of electrospun fibers as solid phase extraction media. Thus, these methods are usually neglected. Enthusiastic readers are referred to Ref. [28] for further information.

Figure 11.4 Structural evolution of polyethylene oxide/chitosan fibers during electrospinning [38].

318

11.3.2

Solid-Phase Extraction

Chemical fiber parameters

Chemistry of electrospun sorbents plays a vital role in its extraction capability, particularly wettability, polarity, and fiber strength. As a result, the chemical characterization of electrospun nanofibers is a crucial step in their preparation.

11.3.2.1 Elemental analysis Elemental analysis can be exploited to understand the basic nature of fibers. Elemental analysis, also known as carbon hydrogen nitrogen sulfur (CHNS) analysis, is a destructive method of choice for fibers with organic backbones. It can determine the percentage of carbon, hydrogen, nitrogen, and sulfur by combustion of nanofibers and subsequent analysis of the gases produced. Ngial et al. prepared electrospun cellulose nanofibers modified with oxolane-2,5-dione for cadmium and lead removal. They calculated the percentage of ligand modification from CHNS analysis [43]. For inorganic fibers, energy dispersive spectroscopy (EDS) is typically used. This method is based on the measurement of the energy and intensity of X-ray photons resulting from the interaction of charged particles (electron or proton) and the fiber. The particles are focused on the surface and the X-rays produced, collected, and energy analyzed. The data can be utilized for quantitative or qualitative analysis of inorganic fibers. In most cases, SEM instruments capable of providing EDS data are used. EDS is nondestructive and produces point-specific elemental analysis. Xia et al. prepared fine composite fibers from a mixture of poly(urethane), poly(methyl methacrylate), and an organic lanthanide complex. The EDS spectra revealed the heterogeneous distribution of the starting materials along the fibers [44]. There are other X-ray-based methods such as X-ray photoelectron spectroscopy (XPS) measuring the kinetic energy and the number of ejected electrons, as a result of the interaction of an X-ray beam with the fiber. The method is surface sensitive and provides information for the outer layers of the surface (w10 nm). The information includes the structural formula and electronic and chemical state at the part per 1000 range. The X-ray-based methods are appropriate for metals or metal oxides composite fibers [45].

11.3.2.2 Chemical bonding To study the chemical bonding of nanofibers, Fourier transform infrared spectroscopy (FTIR) is an appropriate nondestructive technique. Interaction of the infrared beam excites the vibrational states of functional groups. The transmission is recorded as a function of wavenumber. Each functional group absorbs at a characteristic wavenumber that can be assigned to a chemical bond in the nanofibers. Electrospun nanofibers can be simply laid on a KBr disk for analysis. For surface-modified nanofibers, attenuated total reflectance (ATR)-based FTIR is implemented. This method is surface sensitive and the signal intensity is increased by extending the light pathway. Ma et al. functionalized the surface of electrospun poly(sulphone) with methyl methacrylate and a cerium salt. They used ATR-FTIR to verify the grafting of poly(methyl methacrylate) to the poly(sulphone) surface [46]. Polarized FT-IR, in which IR beams

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are aligned in different axis with a polarizer, can provide information about the orientation of molecules, especially when the nanofibers are spatially oriented. Kim et al. prepared randomly deposited and partially aligned electrospun nylon-6 nanofibers by using a rotating collector with different linear velocities. They showed that polarized FTIR data could distinguish between random and aligned fibers [47]. Nuclear magnetic resonance (NMR) spectroscopy provides structural information according to the concept of the chemical shift. The only problem is that electrospun fibers should be dissolved in a deuterated solvent, which makes this method destructive and expensive. Solid-state NMR is also possible for obtaining data without solvent dissolution. Ngila et al. prepared electrospun cellulose acetate fibers and were hydrolyzed confirming the deacetylation process by solid-state NMR [43].

11.3.3 Thermal stability In some cases the thermal stability of electrospun nanofibers would be of great importance especially during extraction processes where thermal desorption or high-temperature extraction is used. Thermogravimetric analysis (TGA) can be used to evaluate the thermal stability of nanofibers. In this method, the sample is exposed to increasing temperature at a known rate, and the amount of weight loss is recorded. Ajji et al. used TGA to select amino-functionalized reduced-graphene oxide (Am-rGO) substrates among other GO derivatives for the preparation of fiber composites. The TGA data (Fig. 11.5A) demonstrated the higher thermal stability of Am-rGO compared with GO and r-GO [48]. Raman spectroscopy can be used to differentiate the difference in amorphous content between the initial precursor and final electrospun product. Using this approach Yang et al. studied the morphology of poly(acrylonitrile) fibers containing graphite crystallites [49]. The left peak in Fig. 11.5B is due to the disordered portion and the right peak the ordered state of the graphitic crystallites. The intensity and ratio of the peaks are related to the heat treatment applied to the sample.

Figure 11.5 (A).TGA curves of rGO, Am-rGO, GO [48]. (B). Raman spectra of PAN-based carbon nanofibers as a function of heat treatment temperature [49].

320

11.4

Solid-Phase Extraction

Electrospun nanofibers types

In recent decades a wide variety of electrospun nanofibers have been synthesized. The electrospinning method (e.g., nozzle or collector format) together with variation in synthetic methods has contributed to the development of nanofibers with unique properties for specific purposes. Some of the recent applications in the field of extraction science are discussed in more detail below.

11.4.1

Molecularly imprinted polymers (MIPs)

Selectivity remains a concern for the developments of target compound extraction using solid phases. Recent decades have witnessed the development of various strategies for building selectivity into the solid phase extraction process. Molecular imprinting approaches are one of the more successful strategies. In this case a monomer and cross-linking agent are mixed in the presence of the target compound to achieve specific molecular recognition. Subsequent removal of the target compound by an appropriate solvent leads to an extraction phase with cavities matched to the shape of the target compound with complementary specific intermolecular interactions. The high specific surface area and low-cost production of electrospun fibers is attractive for the preparation of MIP-based adsorbents. However, a cross-linked polymeric network cannot be electrospun easily due to its insolubility. There are two approaches for producing electrospun MIP nanofibers: imprinting the electrospun fiber or encapsulation of MIP nanoparticles (NPs) into the electrospun nanofibers. In the direct method, an appropriate functional polymer and template molecule are blended and used for electrospinning. The template is then removed and leaves imprinted sites in the electrospun fibers [50]. Using the alternative approach, MIP-NPs are easily encapsulated inside the electrospun nanofibers [51e53]. The recognition sites in MIP-NPs remain intact even after encapsulation and can be used for trace analysis of different template compounds from complex samples [51,54]. Also, the centrifugation step commonly used to separate particles from solution, can be omitted for nanofiber-based adsorbents. The electrospun chiral imprinted membrane for (e)-cinchonidine showed the same target affinity for the immobilized microspheres and the free form [55]. Chronakis and coworkers used poly(ethylene terephthalate) (PET) as the supporting nanofibers matrix for encapsulation of 17 b-estradiol and theophylline imprinted nanoparticles [52]. Yoshimatsu et al. used MIP-NPs in a PET support through electrospinning for the solid phase extraction of propranolol. It was demonstrated that the binding sites in the nanofiber composites remained intact for the chiral-selective extraction of propanolol [51]. The same authors proposed a simple approach employing direct generation of recognition sites during the electrospinning process for the extraction of 2,4-dichlorophenoxyacetic acid (2,4-D) [50]. The synthesized nanofibers provide functional groups that interacted with the template compound during electrospinning. These results demonstrated the successful preparation of robust MIP nanofibers that can selectively rebind the target molecule. Poly(styrene)based MIP nanofibers were synthesized for atrazine as a template via a noncovalent

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approach [56]. To compare the selectivity of the MIP-based nanofibers, a comparison was made with nonimprinted polymer (NIP) nanofibers for the extraction of atrazines from river water. MIP-nanofiber membranes have been used for the selective enrichment of Rhodamine B (RhB) [57], bisphenol A, basic tebuconazole [58] and (e)-cinchonidine [55]. The molecularly imprinted membranes were produced by the electrospinning of RhB MIP microspheres and poly(ethylene terephthalate) as the matrix polymer. The electrospun imprinted polymer membranes demonstrated a higher affinity for the target compounds than molecularly imprinted particles. In addition, the imprinted nanofiber membranes had higher selectivity for the target compounds than nonimprinted analogs [57]. Electrospun molecularly imprinted polymer membranes were also prepared with two types of MIP-NPs as multianalyte selective membranes for the simultaneous isolation of trace amounts of bisphenol A and basic tebuconazole in vegetables and fruit juices [58]. MIP electrospun nanofibers of poly(styrene) were employed as a coating for solid phase microextraction (SPME) for the extraction of parabens from environmental waters [59]. More recently, the MIP methodology was used to produce an extraction material via sol-gel electrospinning. In this process, poly(amide)/tetraethoxyorthosilane (TEOS) solution was electrospun followed by a hydrolysis and condensation step using thermal treatment. After aging the organic backbone was removed by external heating. The imprinted-silica nanofibers were used for the extraction of atrazine in real samples by the m-solid phase extraction technique [60].

11.4.2 Core-shell and hollow fibers Core-shell nanofibers and hollow nanotubes were developed by coaxial electrospinning. The main applications of coelectrospinning consist of encapsulation of different biologically active compounds; formation of nanotubes, cell scaffolds, and drug release [61]. Core-shell nanofibers can also be produced using a single nozzle setup without complex coannular nozzles. Accordingly, Bazilevsky et al. synthesized core-shell nanofibers using an emulsion of two polymer solutions, PMMA/ poly(acrylonitrile) (PAN) in N,N-dimethylformamide (DMF). To this end, PMMA/ DMF and PAN/DMF were mixed and left for 1 day. During this period, PMMA droplets are dispersed in the PAN matrix. Precipitation of PMMA droplets in the PAN solution and its entrapment in the Taylor cone leads to core-shell formation during electrospinning. The use of a single nozzle system relies on the configuration of the large-particle emulsion. An emulsion of PMMA/DMF droplets in PAN/DMF gave multicore-shell nanofibers by electrospinning [14]. Core-shell nanofibers have also been synthesized by a two-step process. Tin oxide (SnO2) was produced by electrospinning and zinc oxide (ZnO) was then deposited by atomic layer deposition (ALD) on the SnO2 nanofibers as a shell layer. These nanofibers have potential for use as gas sensors for oxygen and nitrogen dioxide [62]. Similarly, TiO2e ZnO core-shell nanofibers were synthesized by conventional electrospinning and ALD for sensing oxygen [63]. Bagheri et al. synthesized electrospun core-shell

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Figure 11.6 Schematic diagram of the homemade core-shell electrospinning setup, PBT/PPy hollow nanofiber obtained from core-shell PVP-PBT/PPy nanofibers [17].

nanofibers of poly(vinylpyrrolidone)-poly(butylene terephthalate)/poly(pyrrole) (PVPePBT/PPy) as a sorbent for m-solid phase extraction (m-SPE) of triazine herbicides from aqueous samples and wheat grains (Fig. 11.6). In order to remove the PVP, core-shell PVP-PBT/PPy nanofibers were submerged in water. The higher extraction capability of the hollow nanofibers compared with conventional fibers confirmed their suitability as an adsorbent for solid phase extraction [17]. A poly(pyrrole) functionalized core-shell electrospun nanofiber mat was used in disk-format for extraction of trace polar analytes from environmental water samples. Acid yellow 9, acid orange 7, and metanil yellow with different numbers of sulfonate functional groups were investigated as model analytes. Intercalation of multifunctional PPy in the PA nanofibrous mat led to significantly higher extraction capability for the organic dyes [64]. Tian et al. developed a simple method for fabrication of electrospun PPy hollow fibers with different surface morphologies. First, poly(caprolactone) (PCL) fibers were prepared by electrospinning, and used as a template for in situ polymerization of PPy as a shell layer. Then the PCL was removed and the hollow fibers used as sorbents for SPE of polar compounds. The PPy hollow fibers with a high specific surface area were used in the packed-fiber SPE (PFSPE) format for extraction of two neuroendocrine markers of behavioral disorders (5-hydroxyindole-3-acetic acid and homovanillic acid) [65].

11.4.3

Polymeric nanofibers

The application of electrospun nanofibers as sorbent materials in sample preparation dates back to 2007 [66]. In 2005 Shin et al. reported the use of poly(styrene) nanofibers as a filter to separate water from a water-oil emulsion [67]. This led to various attempts at packing nanofibers into different formats as membranes and packed tips for the extraction of organic compounds from different matrices [68]. PA has been used most widely for extraction purposes. In 2010, PA fibers were packed in a disk in a homemade device for extraction of estrogens from environmental water samples.

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In this study, a comparison between a commercial nylon microporous membrane and octadecylsiloxane-bonded silica (ODS) cartridge was performed [69]. Also, electrospun PA nanofibers were applied for the extraction of phthalate esters by SPE and their performance compared with octadecylsiloxane-bonded silica cartridges. It was demonstrated that nanofibers had a good potential as an efficient SPE sorbent [70]. Docetaxel was extracted by the PA nanofibers (diameters 400e800 nm) from rabbit plasma [71]. Also, three types of nylon 6 nanofibers were prepared with different surface densities (5.04, 3.90, and 0.75 g m
2) and used for solid phase extraction of parabens, steroids, flavonoids, and pesticides. These nanofibers were produced by needleless electrospinning. Their extraction performance was similar to Oasis HLB particle-packed cartridges [72]. In 2014, an online meSPE setup for extraction of clodinafop propargyl was developed by employing a cartridge packed with PA nanofibers as the HPLC sample loop located on a six-port valve. The online extraction was performed by pumping the sample through the cartridge followed by analytes desorption into the HPLC column (Fig. 11.7) [73]. In another study, PA nanofibers were used in headspace solid phase microextraction (HS-SPME) of chlorophenols from aqueous samples using GCeMS. The concentration of polymer in the electrospinning solution had a crucial role in the suitability of the polymer for this analysis [74]. Table 11.1 lists other polymers which have been used to produce electrospun nanofibers.

11.4.4 Copolymers There are two general approaches for surface modification of electrospun polymeric sorbents. One is to synthesize a hydrophilic/hydrophobic copolymer using at least one monomer containing polar functional groups. An alternative route is to modify

Figure 11.7 The online m-SPE of clodinafop propargyl using PA nanofibers [73].

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Table 11.1 Summary of applications of electrospun nanofibers. Extraction technique

Instrumental technique

Analytes

Matrix

Reference

PS

SPME

GC-MS

Pesticides

Honey

[75]

PS

PFSPE

HPLC-UV

Trazodone

Plasma

[66]

PS

DLLME

HPLC-UV

Aldehydes

water

[76]

PS

Semimicro SPE

HPLC-DAD

Steroid compounds

Plasma & water

[68]

PS

PFSPE

LC-MS/MS

Diethylstilbestrol, hexestrol, and dienestrol

Milk

[77]

PS

SPE

UVeVis spectroscopy at lmax ¼ 638 nm

Disulfine Blue

water

[78]

PS

SPE

HPLC-DAD

Microcystins

Water

[79]

PAN

PT-SPE

HPLC-UV

Nitroaromatic compounds

wastewater

[80]

Silk

SPME

GC-FID

Isopropyl alcohol

water

[81]

PET

SPME

UV

Chromium(VI)

water

[82]

PET

SPME

GC-MS

PAHs

water

[83]

PI

TFME

GC-MS

Phenols

water

[84]

PU

SPME

GC-MS

chlorobenzenes

Water

[85]

Solid-Phase Extraction

Polymer

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a hydrophobic substrate by imparting polar moieties into the polymer structure [86]. Poly(styrene), due to its hydrophobic nature, is a suitable substrate for the extraction of moderate and nonpolar compounds but suffers from poor wettability [87]. Addition of surface polar groups can obviate this issue. Styrene-co-methacrylic acid copolymer has been elestrospun to enhance the extraction of aromatic hydrocarbons from water samples [88], as well as the determination of vitamins E and A in plasma samples [89]. Electrospun styrene-co-styrene sodium sulfonate copolymer was used for the extraction of several hormones from water samples. This copolymer demonstrated higher extraction efficiency than poly(styrene) fibers [90]. Ifegwu et al. prepared a styrene-co-methacrylic acid, styrene-co-p-sodium styrene sulfonate, and styrene-coacrylamide polymers for SPE of 1-hydroxypyrene in urine, with a higher yield than styrene-co-acrylamide polymers [91].

11.4.5 Carbon fibers Carbon fibers (CFs) have been developed for a wide range of applications [92]. CNFs can be fabricated by catalytic vapor deposition [93] and electrospinning, especially PAN-based CNFs [94,95]. To fabricate PAN-based CNFs, PAN nanofibers are prepared as a precursor for CNFs production. The properties of the CNFs are controlled by the selected polymer solution and the processing parameters. Once the PAN nanofibers are prepared, heat treatment is applied to the nanofibers [95]. This consists of a two-step stabilization and carbonization process [96]. Stabilization is usually carried out in air at temperatures between 200 and 300 C and carbonization is conducted in an inert atmosphere at 800e2800 C [97,98]. The applicability of activated CNFs was investigated for preconcentration of organophosphorus pesticides (OPPs) by LC-UV [99]. Zewe et al. dissolved SU-8 2100 in cyclopentanone to prepare a polymer solution for electrospinning subsequently used as SPME fibers. The carbon nanofibers-based coating was prepared by pyrolyzing the SU-8 nanofibers. The extraction characteristics of the SU-8 and pyrolyzed electrospun-coated wires were investigated for nonpolar (benzene, toluene, ethylbenzene, and o-xylene) and polar (phenol, 4-chlorophenol, and 4-nitrophenol) compounds for headspace extraction. The extraction efficiencies of the electrospun fibersecoated wires were compared with PDMS and PDMS/DVB fibers for the extraction of BTEX pollutants. The electrospun fibers prepared at temperatures between 600 and 800 C demonstrated enhanced extraction efficiencies compared to the commercial fibers. The extraction performance of the electrospun fibers for SPME was evaluated for the extraction of phenols and provided improved performance compared with commercial poly(acrylate) SPME fibers [100].

11.4.6 Inorganic fibers Inorganic nanofibers can be synthesized by electrospinning via different synthetic routes (Fig. 11.8B). One approach is to add an inorganic precursor, typically a metal halide or a metal alkoxide, to a spinnable polymer solution. These precursors can be chemically transformed to the corresponding metal oxide via sol-gel reactions.

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Figure 11.8 Synthetic concepts for the preparation of electrospun fibers: A: preparation of polymeric fibers; B: preparation of inorganic fibers via the sol-gel route (with or without the addition of organic polymers); C: preparation of inorganic fibers via the nanoparticle approach; D: preparation of inorganic fibers via the sol-gel method and nanoparticle approach [107].

A thermal treatment is then used to remove the polymer by combustion. Sol-gel solutions may become very viscous during gelation of the precursors. During the hydrolysis reaction, a network of metal-oxygen bonds is established, resulting in gel formation (a highly viscous inorganic cross-linked polymer). Depending on the level of condensation, the viscosity of the sol-gel solution changes during the course of the reaction, which can lead to clogging of the electrospinning needle [101]. Another method is based on the use of inorganic nanoparticles rather than sol-gel precursors (Fig. 11.8C). In this strategy, a viscous solution of an organic polymer is combined with a dispersion of inorganic NPs. The difficulties with this approach are, firstly, the availability of suitable dispersible inorganic NPs, and secondly, the preparation of solutions with a certain “threshold” concentration of NPs. This nanoparticle-

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approach has been little investigated compared with the sol-gel approach and is categorized as part of a more general colloidal electrospinning concept, which is mostly used to prepare composite fibers [102]. An advantage of the NP-approach is that materials which cannot be prepared via sol-gel chemistry become applicable. In this case, a synthetic concept, which uses a spinning solution containing NPs and a small amount of sol-gel precursor as “molecular glue,” can be applied (Fig. 11.8D) [103]. Titanium dioxide nanofibers (TDNFs) were prepared by this technique. TiO2/poly(vinyl pyrrolidone) nanofibers were prepared from (PVP)/pluronic123 composite nanofibers by calcining at 450 C in air to remove the organic constituents and produce TDNFs. SPE with titanium dioxide nanofibers was used for the preconcentration of thallium from tea components, such as poly(phenols), soluble sugars, catechin, caffeine, and tea pigments [104]. The fabrication of ceramic nanofibers by electrospinning consists of three main steps. The first is the preparation of a homogenous spinnable polymer solution containing the inorganic precursor. The second step is electrospinning of the solution under controlled conditions, and the final step is calcination of the fibers in a furnace at elevated temperatures to remove the polymer. For this approach, poly(vinylpyrrolidone) is commonly used as the organic component of the sol during the electrospinning process. To improve the adsorption capacity, four different transition metals (Co, Mn, Ni, and Fe) are added to the sol for titanium-based nanofibers. These fibers are calcined at 500 C to remove the PVP. Ceramic nanofiber sheets containing the transition metals, Fe-Mn, Fe-Ni, Fe-Co, and Fe-Mn-Co-Ni were prepared. The electrospun fibers were used for the online extraction of naproxen and clobetasol from plasma and urine by online m-SPE. All ceramic fibers exhibited acceptable efficiencies for the extraction of the target compounds largely independent of the metal incorporated into the fibers [105]. Ordered mesoporous silica fibers (OMSF) can be prepared by electrospinning combined with pseudomorphic synthesis. Initially, amorphous silica fibers (ASF) are fabricated via electrospinning of a PVA/SiO2 composite followed by calcination to remove the organic component. Then the ASF was transformed into an ordered mesoporous phase by pseudomorphic synthesis. The surface area and total pore volume was dramatically increased in comparison with the ASF precursor. The OMSF was used for the preconcentration of endogenous peptides by a lab-in-syringe approach based on hydrophobic interactions and a size-exclusion mechanism [106].

11.4.7 Composites Composite materials consist of two or more phases of different chemical composition, with mechanical and physical properties different from the original constituents. For instance, high strength and resistance to mechanical and thermal shock, and corrosion are the characteristics of many composites. The continuous phase in composites is called the matrix while the other phase is known as the dispersed phase. The usual classification of composite materials based on the type of matrix includes: (i) polymer matrix composites (PMCs); (ii) metal matrix composites (MMCs); (iii) ceramic matrix

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composites (CMCs); and (iv) carbon matrix composites (CAMCs). PMCs are the most widely used composites [108]. Nanofiber composites exhibit larger surface area-tovolume ratios and higher adsorption capacity, making them more suitable as extraction media compared with powder composites [109]. Composite nanofibers are synthesized by two techniques, known as the template method and electrospinning of a composite solution polymer. In the first method, the electrospun polymer fibers as a mat are immersed into a solution containing composite particles which adsorb onto the fiber surface. It is a time-consuming process and the amounts of entrapped materials are uncontrollable. The second method is based on the combination of polymer with composite which reduces the preparation time and results in spun fibers with a defined composition. The latter is widely used as a rapid fabrication process for composite nanofibers [110].

11.4.7.1 Polymer matrix composite nanofibers (PMCNs) PMCNs are the most common subset of composite materials designed to improve the properties of polymeric nanofibers and to introduce desired functionalities into the polymer matrix. These nanofibers have been used for the extraction of pesticides, industrial chemicals, polycyclic aromatic hydrocarbons, pharmaceuticals, heavy metals, and toxic compounds. Metal-organic frameworks (MOFs), graphene, inorganic oxide nanoparticles, and polymers have been used as dispersed phases in PMCNs. MOFs are compounds consisting of metal ions or clusters coordinated to organic ligands to form one-, two-, or three-dimensional (3D) structures. They are amenable to fine tuning of their pore structure appropriate for selective extraction. High back pressure is a challenging when MOF particles are used in SPE cartridges. An effective solution is to incorporate the MOF particles into electrospun nanofibers. Yan et al. investigated the applicability of the electrospun UiO-66/ poly(acrylonitrile) nanofibers as MOF-polymer composite nanofibers for pipette tip-SPE (PT-SPE) of phytohormones in vegetable samples [111]. High extraction efficiency and sensitivity along with excellent reproducibility were obtained for the phytohormones. Spider web-like chitosan/MIL-68(Al) composite nanofibers were developed for the analysis of trace levels of Pb(II) and Cd(II) after SPE using inductively coupled plasma optical emission spectrometry [112]. Water-stable methyl-modified MOF-5/poly(acrylonitrile) composite nanofibers were used for the solid phase extraction of estrogens from urine [113]. The water solubility of MOFs is a general challenge for SPE. MOF-5 and several other MOFs exhibit limited solubility when exposed to water and moisture. The incorporation of hydrophobic functional groups, such as methyl, into the structure of MOF-5, modifies its water solubility. In addition, the combination of modified MOF-5 with poly(acrylonitrile) as a nanofiber composite increases pep interactions, hydrogen bonding, and hydrophobic interactions between the estrogenic drugs and the sorbent. Another limitation of MOFs is their separation from the substrate during thin film microextraction (TFME). To solve this problem, Xu et al. synthesized a poly(styrene)/MOF-199 electrospun nanofiber composite and investigated its use for the extraction of aldehydes from urine [114].

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Graphene (G) is a single layer of carbon atoms arranged in a hexagonal lattice. Its excellent mechanical, thermal, electrical properties and ultrahigh specific surface area make it an interesting material for sample preparation. However, it has some limitation, such as: (i) leakage and blocking when the nanosized graphene is used as a sorbent in SPE and (ii) the tendency to aggregate leading to a decrease in the accessible surface area. These drawbacks were addressed by Xu et al. by incorporation of graphene into PS nanofibers as a thin film sorbent [115]. Acceptable recovery and low LODs demonstrated the suitability of the nanofiber composite for the isolation of aldehydes from complex breath samples. Inorganic oxide NPs are another dispersed phase, which generate nanocomposite structures with nanoscale morphology. The effect of inorganic oxide nanoparticles on the extraction efficiency of electrospun poly(ethylene terephthalate) composites was investigated by Bagheri et al. [116]. Four types of nanoparticles (Fe3O4, SiO2, CoO, and NiO) along with a PET polymer were electrospun and evaluated as SPME fiber coatings. The electrospun SiO2ePET nanocomposite showed superior performance for the extraction of aromatic compounds. The porous structure, high specific surface area, and hydrophobicity of the fiber are probably the major factors contributing to its enhanced performance. Silica supported Fe3O4 nanoparticles (Fe3O4-SiO2) were also used as the dispersed phase for the synthesis of an electrospun nanocomposite based on magnetic nanoparticles-poly(butylene terephthalate) (MNPs-PBT) [117]. The extraction efficiency was studied by online m-SPE-LC-UV (Fig. 11.9A) for the analysis of antiinflammatory and loop diuretics as model analytes. High porosity and paramagnetic properties of the MNPs-PBT nanofibers enhanced the extraction efficiency of the selected drugs. By immersing the diamagnetic compounds into paramagnetic media, the analytes are more willing to accumulate in the areas in which the minimum magnetic field is applied. In another study, Feng et al. developed a novel sorbent by immobilizing oxidized carbon nanotubes (OCNTs) in PS and used the PS/OCNTs film as TFME adsorbent for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDIeTOFeMS) (Fig. 11.9B) [118]. The PS/OCNTs film demonstrated a

Figure 11.9 (A) The schematic diagram of the magnetic field assisted online m-SPE-LC-UV [117] (B) the Schematic diagram of the TFME technique coupled with MALDI-TOF-MS analysis using an electrospun PS/OCNTs film [118].

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favorable adsorption capacity for benzo[a]pyrene and 1-hydroxypyrene from urine. In addition, the PS/OCNTs film was an excellent substrate for MALDI. Various polymers can also be dispersed into the polymer matrix to produce composites with diverse properties, also called hybrid materials. Many reports describe the use of polymer-polymer composite nanofibers as sorbent s for various SPE and SPME formats. For instance, a poly(aniline)-nylon-6 (PANI-N6) nanofiber sheet was employed for headspace adsorptive microextraction of aromatic compounds (such as chlorobenzenes) from different environmental water samples by Bagheri et al. [119]. Hydrophilic nanofibers synthesized by the in-situ formation of Ag NPs on poly(dopamine) coated polystyrene electrospun fibers (PS@PDA-Ag) were utilized as m-SPE sorbent for the online determination of PAHs in human urine [120]. The large surface area-to-volume ratio, high porosity, and resistance to matrix fouling are the attractive properties of this nanofiber sorbent. A PS/G nanofiber membrane was immersed in a dopamine solution for autopolymerization [121]. Then, the core-sheath conjugated electrospun PS/G@PDA nanofibers were used for the extraction of aldehyde metabolites in human urine. Ebrahimzadeh et al. also reported a reasonable approach to enhance the extraction capability of electrospun nanofibers [122]. A composite solution of poly(4-nitroaniline) (P4-NA) and PVA was electrospun and the porosity of the nanofibers increased by dissolving away the PVA fraction in hot water. The resulting nanofibers exhibited satisfactory performance for the extraction of organophosphorus pesticides from aqueous solution by SPME. Satínský et al. described an innovative approach for the preparation of nanofiber composites using a combination of melt-blown and electrospinning [123]. The combination of two different procedures created a stable 3D porous structure suitable for online SPE-LC system at high pressures. The extraction efficiency of the poly(caprolactone) and poly(vinylidene fluoride) composite nanofibers (PCL/PVDF) was compared with the 2D mat nanofibers. The 3D-based structure was shown to be suitable for the online extraction of ochratoxin A from beer samples.

11.4.7.2 Ceramic matrix composite nanofibers (CMCNs) The crystalline structure and strong atomic bonds in ceramic nanofibers impart excellent properties such as hardness, compressive strength, thermal and chemical stability, and corrosion resistance. However, the brittle nature, low toughness, poor tensile strength, and lack of flexibility limit the application of the pure ceramic nanofibers. To overcome these issues, ceramic composite nanofibers have been developed with improved surface area and porosity [124]. Bagheri et al. evaluated Ti-Fe-Mn, Ti-Fe-Ni, Ti-Fe-Co and Ti-Fe-Mn-Co-Ni ceramic nanofibers as sorbent for the extraction of naproxen and clobetasol using a m-SPE setup online with HPLC [105]. The porous structure and high aspect ratio of the ceramic composite nanofibers indicated their potential for use in the cleanup and preconcentration of naproxen and clobetasol from urine and blood plasma samples.

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11.4.7.3 Carbon matrix composite nanofibers (CAMCNs) The large surface area and high chemical stability of CAMCNs are ideal features for their applications as sorbents. However, their application as extractive media has not been explored much. The applicability of electrospun CAMCNs as an extractive phase for the preconcentration of aniline compounds was reported by Basheer et al. [125]. The carbon nanofibers were obtained from carbon soot by combusting natural oil. These were mixed with PVA and electrospun to synthesize the CAMCNs mat. The extraction ability of the CAMCNs membrane in m-SPE format was evaluated by HPLC.

11.4.8 Three-dimensional (3D) electrospun nanofibers Three-dimensional electrospun nanofibrous scaffolds have many potential applications in different fields such as tissue engineering, solar cells, filters, and energy storage. 3D nanofibers can be synthesized by combining liquid-assisted collection in the electrospinning process, called wet electrospinning, without resorting to harsh chemical or experimental conditions. The nanofiber mats are expected to have relatively larger surface areas and higher porosity compared with 2D mats [126]. The first application of 3D electrospun nanofibers for the extraction of chlorobenzenes (CBs) was reported, recently [127]. Fig. 11.10 illustrates the wet electrospinning setup for the synthesis of 3D nanofibrous scaffolds and microoriented extraction setup, schematically. The comparison of 3D scaffolds and 2D nanofibers mats as an extractive phase in needle trap microextraction (NTME) of CBs demonstrates their superiority over conventional nanofibers. The induction of the third dimension has a surprising effect on extraction efficiency as the enhancement of the extraction yield is considerable. The interesting features of 3D nanofibrous scaffolds and promising results verifies their suitability as extractive phases in different extraction methods such as SPME, NTME, and SPE.

Figure 11.10 (A) The schematic diagrams of wet electrospinning used for preparation of 3D polyamide scaffolds and (B) microoriented extraction setup [127].

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[75] Zali S, Jalali F, Es-Haghi A, Shamsipur M. Electrospun nanostructured polystyrene as a new coating material for solid-phase microextraction: application to separation of multipesticides from honey samples. J Chromatogr B 2015;1002:387e93. [76] Liu F, Song D, Huang X, Xu H. Electrospun polystyrene nanofibers as a novel adsorbent to transfer an organic phase from an aqueous phase. J Sep Sci 2016;39(7):1326e30. [77] Hu W-Y, Kang X-J, Zhang C, Yang J, Ling R, Liu E-H, et al. Packed-fiber solid-phase extraction coupled with high performance liquid chromatographyetandem mass spectrometry for determination of diethylstilbestrol, hexestrol, and dienestrol residues in milk products. J Chromatogr B 2014;957:7e13. [78] Hashemifard N, Shariati S. Electrospun polystyrene nanofiber as an adsorbent for solidphase extraction of disulfine blue from aqueous samples. Arabian J Sci Eng 2016;41(7): 2487e92. [79] Wei H, Yang F, Wang Y, Zhou Y, Yan Y, Liang G, et al. Electrospun polymer nanofibres as solid-phase extraction sorbents for extraction and quantification of microcystins. Environ Technol 2015;36(21):2796e802. [80] Tavengwa NT, Nyamukamba P, Cukrowska E, Chimuka L. Miniaturized pipette-tipbased electrospun polyacrylonitrile nanofibers for the micro-solid-phase extraction of nitro-based explosive compounds. J Sep Sci 2016;39(24):4819e27. [81] M€uller V, Cestari M, Palacio SM, de Campos SD, Muniz EC, de Campos EA. Silk fibroin nanofibers electrospun on glass fiber as a potential device for solid phase microextraction. J Appl Polym Sci 2015;132(13). [82] Sereshti H, Amini F, Najarzadekan H. Electrospun polyethylene terephthalate (PET) nanofibers as a new adsorbent for micro-solid phase extraction of chromium (VI) in environmental water samples. RSC Adv 2015;5(108):89195e203. [83] Bagheri H, Akbarinejad A, Aghakhani A. A highly thermal-resistant electrospun-based polyetherimide nanofibers coating for solid-phase microextraction. Anal Bioanal Chem 2014;406(8):2141e9. [84] Li S, Wu D, Yan X, Guan Y. Acetone-activated polyimide electrospun nanofiber membrane for thin-film microextraction and thermal desorption-gas chromatographye mass spectrometric analysis of phenols in environmental water. J Chromatogr A 2015; 1411:1e8. [85] Bagheri H, Aghakhani A. Novel nanofiber coatings prepared by electrospinning technique for headspace solid-phase microextraction of chlorobenzenes from environmental samples. Anal Method 2011;3(6):1284e9. [86] Fontanals N, Galia M, Marcé RM, Borrull F. Solid-phase extraction of polar compounds with a hydrophilic copolymeric sorbent. J Chromatogr A 2004;1030(1e2):63e8. [87] Kang XJ, Chen LQ, Zhang YY, Liu YW, Gu ZZ. Performance of electrospun nanofibers for SPE of drugs from aqueous solutions. J Sep Sci 2008;31(18):3272e8. [88] Qi D, Kang X, Chen L, Zhang Y, Wei H, Gu Z. Electrospun polymer nanofibers as a solid-phase extraction sorbent for the determination of trace pollutants in environmental water. Anal Bioanal Chem 2008;390(3):929e38. [89] Liu Z, Kang X, Fang F. Solid phase extraction with electrospun nanofibers for determination of retinol and a-tocopherol in plasma. Microchim Acta 2010;168(1e2):59e64. [90] Zhang Y, Kang X, Chen L, Pan C, Yao Y, Gu Z-Z. Fiber-packed SPE tips based on electrospun fibers. Anal Bioanal Chem 2008;391(6):2189e97. [91] Ifegwu OC, Anyakora C, Chigome S, Torto N. Electrospun nanofiber sorbents for the pre-concentration of urinary 1-hydroxypyrene. J Anal Sci Tech 2015;6(1):14.

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[92] Lu W, Zu M, Byun JH, Kim BS, Chou TW. State of the art of carbon nanotube fibers: opportunities and challenges. Adv Mater 2012;24(14):1805e33. [93] Tibbetts GG, Lake ML, Strong KL, Rice BP. A review of the fabrication and properties of vapor-grown carbon nanofiber/polymer composites. Compos Sci Technol 2007;67(7e8): 1709e18. [94] Qin X-H, Wan Y-Q, He J-H, Zhang J, Yu J-Y, Wang S-Y. Effect of LiCl on electrospinning of PAN polymer solution: theoretical analysis and experimental verification. Polymer 2004;45(18):6409e13. [95] Kalayci VE, Patra PK, Kim YK, Ugbolue SC, Warner SB. Charge consequences in electrospun polyacrylonitrile (PAN) nanofibers. Polymer 2005;46(18):7191e200. [96] Feng L, Xie N, Zhong J. Carbon nanofibers and their composites: a review of synthesizing, properties and applications. Materials 2014;7(5):3919e45. [97] Zussman E, Chen X, Ding W, Calabri L, Dikin D, Quintana J, et al. Mechanical and structural characterization of electrospun PAN-derived carbon nanofibers. Carbon 2005; 43(10):2175e85. [98] Wu M, Wang Q, Li K, Wu Y, Liu H. Optimization of stabilization conditions for electrospun polyacrylonitrile nanofibers. Polym Degrad Stabil 2012;97(8):1511e9. [99] Maddah B, Soltaninezhad M, Adib K, Hasanzadeh M. Activated carbon nanofiber produced from electrospun PAN nanofiber as a solid phase extraction sorbent for the preconcentration of organophosphorus pesticides. Separ Sci Technol 2017;52(4):700e11. [100] Zewe JW, Steach JK, Olesik SV. Electrospun fibers for solid-phase microextraction. Anal Chem 2010;82(12):5341e8. [101] Ramaseshan R, Sundarrajan S, Jose R, Ramakrishna S. Nanostructured ceramics by electrospinning. J Appl Phys 2007;102(11):7. [102] Wessel C, Ostermann R, Dersch R, Smarsly BM. Formation of inorganic nanofibers from preformed TiO2 nanoparticles via electrospinning. J Phys Chem C 2010;115(2):362e72. [103] Horzum N, Muneoz-Espí R, Glasser G, Demir MM, Landfester K, Crespy D. Hierarchically structured metal oxide/silica nanofibers by colloid electrospinning. ACS Appl Mater Interfaces 2012;4(11):6338e45. [104] Chen S, Yan J, Li J, Zhang Y, Lu D. Solid phase extraction with titanium dioxide nanofibers combined with dispersive liquid-liquid microextraction for speciation of thallium prior to electrothermal vaporization ICP-MS. Microchim Acta 2017;184(8): 2797e803. [105] Bagheri H, Piri-Moghadam H, Rastegar S, Taheri N. Electrospun titania solegel-based ceramic composite nanofibers for online micro-solid-phase extraction with highperformance liquid chromatography. J Sep Sci 2014;37(15):1982e8. [106] Zhu G-T, Li X-S, Fu X-M, Wu J-Y, Yuan B-F, Feng Y-Q. Electrospinning-based synthesis of highly ordered mesoporous silica fiber for lab-in-syringe enrichment of plasma peptides. Chem Commun 2012;48(80):9980e2. [107] Wessel C. Nanostructured mesoporous materials via electrospinning: principle concepts in the preparation of oxide nanofibers from different building blocks. 2016. [108] Kutz M. Mechanical engineers’ handbook. In: Materials and engineering mechanics, vol. 1. John Wiley & Sons; 2015. [109] Asiabi M, Mehdinia A, Jabbari A. Electrospun biocompatible Chitosan/MIL-101 (Fe) composite nanofibers for solid-phase extraction of D 9-tetrahydrocannabinol in whole blood samples using Box-Behnken experimental design. J Chromatogr A 2017;1479: 71e80.

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[110] Sawicka KM, Gouma P. Electrospun composite nanofibers for functional applications. J Nanoparticle Res 2006;8(6):769e81. [111] Yan Z, Wu M, Hu B, Yao M, Zhang L, Lu Q, et al. Electrospun UiO-66/polyacrylonitrile nanofibers as efficient sorbent for pipette tip solid phase extraction of phytohormones in vegetable samples. J Chromatogr A 2018;1542:19e27. [112] Asiabi M, Mehdinia A, Jabbari A. Spider-web-like chitosan/MIL-68 (Al) composite nanofibers for high-efficient solid phase extraction of Pb (II) and Cd (II). Microchim Acta 2017;184(11):4495e501. [113] Asiabi M, Mehdinia A, Jabbari A. Preparation of water stable methyl-modified metale organic framework-5/polyacrylonitrile composite nanofibers via electrospinning and their application for solid-phase extraction of two estrogenic drugs in urine samples. J Chromatogr A 2015;1426:24e32. [114] Liu F, Xu H. Development of a novel polystyrene/metal-organic framework-199 electrospun nanofiber adsorbent for thin film microextraction of aldehydes in human urine. Talanta 2017;162:261e7. [115] Huang J, Deng H, Song D, Xu H. Electrospun polystyrene/graphene nanofiber film as a novel adsorbent of thin film microextraction for extraction of aldehydes in human exhaled breath condensates. Anal Chim Acta 2015;878:102e8. [116] Bagheri H, Roostaie A. Roles of inorganic oxide nanoparticles on extraction efficiency of electrospun polyethylene terephthalate nanocomposite as an unbreakable fiber coating. J Chromatogr A 2015;1375:8e16. [117] Bagheri H, Khanipour P, Asgari S. Magnetic field assisted m-solid phase extraction of anti-inflammatory and loop diuretic drugs by modified polybutylene terephthalate nanofibers. Anal Chim Acta 2016;934:88e97. [118] He X-M, Zhu G-T, Yin J, Zhao Q, Yuan B-F, Feng Y-Q. Electrospun polystyrene/ oxidized carbon nanotubes film as both sorbent for thin film microextraction and matrix for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. J Chromatogr A 2014;1351:29e36. [119] Bagheri H, Aghakhani A. Polyaniline-nylon-6 electrospun nanofibers for headspace adsorptive microextraction. Anal Chim Acta 2012;713:63e9. [120] Zhang H, Xu H. Electrospun nanofibers-based online micro-solid phase extraction for the determination of monohydroxy polycyclic aromatic hydrocarbons in human urine. J Chromatogr A 2017;1521:27e35. [121] Zhang H, Hu S, Song D, Xu H. Polydopamine-sheathed electrospun nanofiber as adsorbent for determination of aldehydes metabolites in human urine. Anal Chim Acta 2016;943:74e81. [122] Mehrani Z, Ebrahimzadeh H, Aliakbar AR, Asgharinezhad AA. A poly (4-nitroaniline)/ poly (vinyl alcohol) electrospun nanofiber as an efficient nanosorbent for solid phase microextraction of diazinon and chlorpyrifos from water and juice samples. Microchim Acta 2018;185(8):384.  [123] Hakova M, Havlíkova LC, Chvojka J, Erben J, Solich P, Svec F, et al. A comparison study of nanofiber, microfiber, and new composite nano/microfiber polymers used as sorbents for on-line solid phase extraction in chromatography system. Anal Chim Acta 2018;1023:44e52. [124] Rana D, Ramalingam M. Ceramic nanofiber composites. In: Nanofiber composites for biomedical applications. Elsevier; 2017. p. 33e54.

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~ pez-Lorente, R. Lucena, S. Cardenas M.C. Díaz-Lin an, A.I. Lo Departamento de Química Analítica, Instituto Universitario de Investigacion en Nanoquímica (IUNAN), Edificio Marie Curie (anexo), Campus de Rabanales, Universidad de Cordoba, Cordoba, Spain

12.1

Introduction

Membranes have been extensively used at the industrial scale as, in general, they act as physical barriers which, based on permeability or size cut-off criteria allow the transfer of gas, liquid, or particulate material between two, usually miscible, phases. Therefore, the membrane can participate as an active or passive element with a common role in facilitating the contact of both phases at the membrane interface. Membranes are employed in sample treatment, either for analyte preconcentration or sample cleanup. Moreover, they can be used in the microextraction context to prevent damage or blockage of the sorbent phase when dirty samples are processed. A paradigmatic example is hollow fiber protected liquid phase microextraction [1]. No doubt, the most interesting application is the use of membranes for analyte enrichment. In this case, target compounds migrate from the sample matrix to the membrane by a chemical, pressure, or electrical field gradient [2]. The so-called hollow fiber liquid phase microextraction (HF-LPME) is identified as a reference technique in microextraction [3]. It can be used in the two-phase (2D) or three-phase (3D) modes. 2DHF-LPME uses two immiscible phases, with the extractant located in the lumen of the fiber. In 3D-HF-LPME the sample and the extractant are miscible (generally aqueous) while the pores of the HF are filled with an organic solvent or solution. The analytes are first extracted into the organic phase, and then transferred to the aqueous solution inside the HF. A chemical mechanism, usually a pH gradient, is used to avoid the reextraction of the analytes to the external aqueous matrix. However, the advantages of using planar formats, mainly related to the higher surface area that can be exposed to the sample, have gained recognition in recent years. Therefore, to increase both the variety of chemistries available for extraction and the membrane capacity and selectivity, there is great interest in developing membranes incorporating nanoparticles and nanostructures. The potential of these materials in solid-phase microextraction is well-known [4]. These outstanding properties, however, are dramatically decreased if the nanoscale is lost. This is common for hydrophobic nanoparticles, especially carbon-based nanoparticles. To avoid this inconvenience, carbon nanoparticles are either dispersed in a micellar media or immobilized on a solid surface. The addition of nanoparticles into a membrane is a synergic combination as, on the one hand aggregation is avoided and, on the other, new capabilities including higher surface area are conferred to the Solid-Phase Extraction. https://doi.org/10.1016/B978-0-12-816906-3.00012-1 Copyright © 2020 Elsevier Inc. All rights reserved.

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membrane. A clear precursor of this concept is disks-based solid-phase extraction, where the sorbent phase is placed in a planar element to minimize channeling during sample processing steps. In addition, higher flow rates can be used, thus allowing the preconcentration of large sample volumes in a reasonable time. If a low eluent volume is used, the enrichment factor that can be achieved leads to improved method sensitivity. There are many membrane formats which can be loaded with (nano)particles, namely planar and hollow fiber membranes, and membranes fabricated via electrospinning and thin films, among others. This chapter provides an overview of the use of particle-loaded membranes in sample treatment. The first section describes the classical approach using membrane formats with a hollow fiber or planar geometry. In the second part, the potential of thin film microextraction as an extension of the particle-loaded membrane concept is described.

12.2 12.2.1

Membranes modified with nanoparticles Hollow fiber membranes

Polypropylene hollow fibers (HF) provide an excellent support for the confinement of the extraction solvent in either 2D or 3D formats. However, considering the small dimensions of HF microextraction units, and thus the small volume of extraction solvents, in the low microliter range, the efficiency of the process can be jeopardized. If an HF is intended to be used in micro solid phase extraction, a similar limitation would appear unless highly efficient sorbents, such as nanostructured solids, are used. Carbon nanoparticles in general, and carbon nanotubes (CNTs) in particular, are by far the preferred nanomaterials for the construction of so-called reinforced HF on account of their outstanding sorbent capacity. These nanoparticles can be held in the lumen or immobilized into the pores of the HF, which eliminates the aggregation typically observed in other micro solid phase extraction formats, such as dispersive extraction techniques. The location of the CNTs inside the HF usually requires the preparation of a composite material using sol-gel technology. Eshaghy et al. prepared an organic/inorganic polymer containing oxidized multiwalled carbon nanotubes (o-MWNTs) in which orthosilicate was used as precursor and HCl as catalyst, for the in situ gelation inside the HF [5]. The reinforced-HF was immersed directly into the sample solution for the extraction, and the porosity of the HF allows fast resorption of the analytes. The fiber also prevents large molecules from reaching and diminishing the adsorption capacity of the extraction phase for the analytes. The fiber was discarded after each extraction. A simplification of this procedure was proposed by Song et al. by replacing the sol-gel approach with a surfactant hexadecyltrimethylammonium bromide (CTAB) [6]. No interference was observed for the extraction of carbamate pesticides from fruits [6] or strychnine and brucine in clinical samples [7]. If CNTs are dispersed in a liquid phase, the sealing of the fiber end is required to avoid material losses [8]. As an alternative, the dispersion of the CNTs either in 1-octanol [9] or water [10] can be added to HF, and external energy (ultrasounds or pressure) used to force the nanomaterial into the wall pores of the HF. In this case,

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a sort of CNTs-supported liquid membrane is obtained. The role of the CNTs is to increase the surface area of the extraction phase and also to increase the partition constant, especially if a 3D configuration is used. Concerning other carbon nanoparticles, Jiménez-Soto et al. evaluated the use of oxidized single-walled carbon nanohorns (o-SWNHs) as the sorbent phase for 2D reinforced HF microextraction [11]. SWNHs are conical-ended carbon nanoparticles of smaller dimensions than CNTs capable of generating stable aggregates with a dahlia shape. These aggregates are still in the nanometric range (less than 100 nm diameter) and therefore maintain the extraction capacity associated with the nanoworld. The o-SWNHs were immobilized on the pores of the HF. An inserted stainless steel wire maintains the rigidity of the fiber allowing its direct immersion in the sample. Triazine herbicides were determined in waters with a sensitivity comparable to, or better than, other microextraction approaches designed for the same analytical problem. It was possible to reuse the fiber up to 30 times. Graphene (G), or its oxidized form (GO), exhibits an enhanced adsorption capacity for many compounds. Graphene was used as a dispersion in 1-octanol in the lumen of an HF membrane for the extraction of carbamate pesticides with better detection limits than those provided when CNTs were used [12]. GO is more suitable for the extraction of inorganic ions and was used for the extraction of heavy metals from waters by a hollow fiber membrane containing a composite adsorbent prepared from GO nanoparticles and silica. The hydrophilic interactions (electrostatic and coordination) that can be established between the oxygenated groups and the metals were identified as the driving force of the extraction [13].

12.2.2 Planar membranes This section is focused on planar membranes modified with (nano)particles. Most of the functionalized membranes described within this section are commercially available and based on different polymers (e.g., nylon, poly(ethylene terephthalate) (PET), poly(vinylidene fluoride) (PVDF), among others), in which the particles have been incorporated via different routes, for example, dip coating, drop-casting directly on the membrane or after a membrane functionalization, that is, amino, carboxylic, etc. Different types of (nano)particles can be loaded onto the membranes, such as metal nanoparticles (Au, Ag, Pd, etc.) as well as some semiconductor nanoparticles (SiO2, Al2O3, etc.) In addition, carbon nanomaterials have also been used as membrane coatings. The incorporation of (nano)particles into membranes can respond to two general needs: (i) improvement of the adsorption/filtration process; and (ii) assist in the subsequent elution and/or detection procedure. For example, nanoparticles can provide interaction sites due to their affinity toward the analyte. In this regard, nylon membranes impregnated with metallic nanoparticles, namely Au and Ag, have been used for the solid phase extraction of Hg by filtration of the water through the membrane [14]. The large surface area of the nanoparticles is responsible for the highefficiency adsorption of Hg into the membrane. Also, Pd nanoparticles have been deposited by dip coating on pore walls of carboxylic and amino group functionalized

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Solid-Phase Extraction C

C

C

C Carboxylation

C

C C C C C C C

C C C C

Amination

C C C C

C A A C A A

A C A C

A C C A A C C A

Immersed in Pd nanoparticle solution

Pristine PET Membrane C -:–COOH

Carboxylated PET Membrane A -:

O

H

C

N ––(CH2–CH2–NH)–n– CH2–CH2–NH2

Aminated PET Membrane : Pd Nanoparticle

Figure 12.1 Scheme of the functionalization process of the poly(ethylene terephthalate) membrane with carboxylic and amino groups and their effect on the immobilization of Pd NPs. Reproduced from Awasthi K, Choudhury S, Komber H, Simon F, Formanek P, Sharma A, et al. Functionalization of track-etched poly (ethylene terephthalate) membranes as a selective filter for hydrogen purification. Int J Hydrogen Energy 2014;39:9356e9365 with permission from Elsevier.

poly(ethylene terephthalate) membranes [15]. The functionalization was critical for the subsequent immobilization of the Pd nanoparticles (Fig. 12.1). These membranes were used in hydrogen separation and sensing. Furthermore, nanosilica extracted from rice straw was combined with a polyether-polyamide block copolymer (PEBA) to form a nanocomposite polymeric membrane for the separation of CO2 in gas streams [16]. Huang et al. prepared PVDF/SiO2 hybrid membranes by a sol-gel process for the preconcentration of fennel oil from herbal water extracts [17]. The presence of SiO2 nanoparticles combined with PVDF resulted in an improvement of the thermal and mechanical properties of the membrane. Zhang et al. [18] developed a micro-solid phase extraction method for food colorants based on the impregnation of a nylon filter membrane with Al2O3 NPs. The performance of the Al2O3 NPs was compared with SiO2 and TiO2 NPs and conventional sorbents such as octadecylsiloxane-bonded silica for the extraction of tartrazine and Sunset yellow in food. The membrane impregnated with Al2O3 NPs exhibited the best extraction efficiency. In addition, solid phase micromembrane tip extraction using 1-butyl-3-methylimidazole iron nanoparticles enclosed within a membrane followed by chiral liquid chromatography was used to evaluate the

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stereoselective interactions of profens with proteins [19], as well as for the analysis of vitamin B in human plasma [20]. Wang et al. prepared a “pH-paper-like” chip for the enrichment and detection of organic pollutants[21]. The chip was composed of a PVDF membrane with a polydopamine (PDA) film and AuNPs. Each component of the chip membranes has a specific role, the PVDF membrane and PDA facilitating the adsorption of organic analytes via hydrophobic interactions and p-p stacking, while AuNPs facilitated the subsequent quantification of the analytes via surface-assisted laser desorption/ionization timeof-flight mass spectrometry. Metallic NPs are used extensively as a substrate in surface enhanced Raman spectroscopy (SERS) due to their capability to enhance the Raman signal. Typical substrates are based on rigid solid materials such as silicon, glass, etc.; however, recently SERS active membranes based on flexible materials have also been described. For example, liquid-crystal polymer (LCP) textile fibers decorated with AgNPs supported by a polyamide filter were used for the extraction and detection of the pesticide thiram [22]. This is an example of the role of metal nanoparticles on the subsequent detection step. Nevertheless, the NPs can play a dual role by supplying additional interaction sites for adsorption of the analytes, for example, via the binding affinity of gold toward sulfur or silver toward amine groups. Granger et al. developed a SERS-based point-of-need diagnostic test for alphafetoprotein (AFP) screening using solid phase microextraction membranes with functionalized AuNPs as Raman enhancer and selective tag for capturing AFP [23]. This field is very extensive and a more detailed description would go beyond the scope of this chapter [24]. However, particle-loaded membranes are expected to have a significant impact on the coupling of extraction with Raman and related detection methods. As well as optical sensors, metallic NPs loaded membranes are used in electrochemical devices, for example, AuNPs formed in situ in a polymer inclusion membrane were used for the electrode immobilization of anti-Salmonella antibodies [25]. Dispersions of MWCNTs and single layer graphene (SLG) nanoparticles have been incorporated into cellulose triacetate polymer matrix membranes [26]. The membranes are prepared via drop-casting of a cellulose triacetate (CTA) polymer matrix and carbon nanomaterials dispersed in dichloromethane on a flat glass surface. Carbon nanomaterials enable the extraction of aromatic compounds, for example, polycyclic aromatic hydrocarbons in sewage pond water samples. One of the challenges to overcome in membrane-based extraction is the specific recognition properties of the membranes as well as their permeation performance. This led to the development of molecularly imprinted membranes (MIMs), which combine molecularly imprinted polymers with membrane separation techniques. Altintas et al. reported the fabrication of MIMs as filters coupled to SPE for the evaluation of water purification processes [27]. MIMs were formed by molecularly imprinted polymer nanoparticles (MIPNPs) toward pharmaceuticals, which were applied onto plasma-treated PVDF membranes (see Fig. 12.2). In addition, the synthesis of molecularly imprinted nanocomposite membranes (MINCMs) has also been described [28]. Nanocomposite membranes were prepared by infiltrating the functional monomer modified with the NPs onto a polydopamine-modified regenerated

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Solid-Phase Extraction

Figure 12.2 Development of functionalized nanostructured polymeric membranes with MIPNPs from PVDF membranes for water purification of pharmaceuticals and subsequent HPLC analysis. Reproduced from Altintas Z, Chianella I, Ponte GD, Paulussen S, Gaeta S, Tothill IE. Development of functionalized nanostructured polymeric membranes for water purification. Chem Eng J 2016;300:358e366 with permission from Elsevier.

cellulose membrane, followed by an imprinting procedure. Imprinted PDA@SiO2 MINCMs for the selective extraction of m-cresol was reported in which the high surface-to-volume ratio and, thus, the large surface area of the SiO2 nanoparticles played an important role in the extraction of the target analyte [29].

12.3

Particle-loaded membranes in a thin film microextraction format

Thin film microextraction (TFME) is a miniaturized sample preparation technique where a flat film of a high surface area-to-volume ratio is used as the extraction phase [30]. Originally proposed in 2001 [31], as an alternative approach to in-fiber SPME, it has continued to evolve with an expanding number of applications. Among the proposed formats, this chapter is focused only on thin films modified with nanoparticles in a membrane configuration generally prepared by dip coating and electrospinning techniques. Readers interested in different coating technologies, theoretical aspects, and applications are referred to more detailed reviews [30,32].

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12.3.1 Nanoparticle-loaded membranes synthesized via electrospinning Electrospinning is a simple, versatile, and widely used technique to synthesize fibrous materials with a variety of diameters, morphologies, and compositions. Briefly, the electrospinning technique consists of a viscous polymeric solution to which a high voltage is applied, after which the charged polymer ejects from the solution toward the collector wire as a fibrous filament [33]. This phenomenon is based on the repulsive electrostatic force that overcomes the surface tension of the polymer when the high voltage is applied. Different electrospinning procedures, for example, magnetic fielde assisted electrospinning, melt electrospinning, or bilayer electrospinning, have been described for the synthesis of nanofibers [34]. An advantage of electrospinning is that the resulting material can be deposited uniformly on a flat surface, thus forming a thin film suitable as an extraction material for TFME or SPME, among others. Moreover, the resulting fibers possess a high surface area, porous structure, and flexibility, desirable for extraction techniques [35]. Sometimes, the electrospinning process is followed by secondary steps, such as reactions for surface modification and other procedures. The incorporation of nanomaterials into electrospun fibers affords different characteristic properties compared with the starting materials; in particular, the surface areato-volume ratio related to the adsorption capacity and in specific cases the adsorption selectivity. For instance, among magnetic nanoparticles, Fe3O4 nanoparticles have acquired special attention in the electrochemical field due to their unique properties, such as excellent conformation stability, better contact between a biocatalyst and its substrate, ease of preparation, and large surface area, among others [36]. The incorporation of nanoparticles into the fiber structure can be carried out by different procedures. Fig. 12.3 illustrates two methods employed to immobilize nanoparticles in electrospun nanofibers, that is, dispersing the nanoparticles into the precursor solution of the polymer or through a chemical procedure in which the fibers are modified to immobilize the nanoparticles into its structure. Both approaches will be discussed in this section.

12.3.1.1 One pot synthesis of electrospun fibers with embedded nanoparticles This approach is the fastest and easiest way to incorporate nanomaterials into electrospun fibers. It consists of the addition of the nanoparticles into the solution of monomers, prior to polymerization. When the nanoparticles are dispersed in the precursor solution of the polymer, they are easily incorporated to the fibers compared to the chemical modification of the fibers with the nanoparticles after electrospinning [34]. Bagheri et al. synthesized electrospun poly(butylene terephthalate) (PBT) nanofibers with embedded magnetic nanoparticles for the isolation of triazines [37]. The inorganic nanoparticles increased the surface area and porosity of the sorbent, thus leading to higher extraction efficiency. Furthermore, since these nanoparticles have magnetic properties, the nanofibers

348 Solid-Phase Extraction

Figure 12.3 Scheme of the alternatives employed to synthesize electrospun fibers with embedded nanoparticles (A) by dispersing the nanoparticles into the precursor solution and (B) by chemical modification of the electrospun fibers and immobilization of nanoparticles.

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could be easily isolated from solution by an external magnet once the microextraction is complete. Although electrospinning provides long synthetic fibers, these can be organized into a membrane format, especially useful for TFME. Liu and Xu obtained electrospun polystyrene/metal-organic frameworks-199 (PS/MOF-199) nanofiber film for TFME of aldehydes in human urine [38]. For the collection of the nanofiber, aluminum foil was employed. MOFs properties, such as tunable pore size, high surface area, and good thermal stability, enhanced the extraction efficiency of the PS/MOF-199 nanofiber film, compared with poly(styrene) films and an MOF-199 coated mesh. Furthermore, no loss of MOF-199 nanoparticles was found during the microextraction procedure, which demonstrates the enhanced mechanical stability of the material. Aldehydes were selected as model analytes, since they are considered to be potential biomarkers of lung cancer. Thus, this method was successfully applied for the determination of aldehydes in urine samples of eight healthy subjects and seven lung cancer patients. Wu et al. synthesized nanofibrous membranes with multianalyte selectivity by encapsulating two types of molecularly imprinted polymer nanoparticles (MIPNPs) into electrospun poly(vinyl alcohol) nanofibers for the simultaneous extraction of bisphenol A and tebuconazole in vegetable and juice samples [39]. Nanofibrous molecularly imprinted membranes (nano-MIMs) retain the advantages of MIP-NPs, such as high molecular selectivity, high binding capacity, and fast binding kinetics, attributed to the large surface-to-volume ratio and the high number of recognition sites at the surface of the core-shell material. In addition, due to their geometric features MIP-NPs provide better accessibility to recognition sites for the analyte as well as lower mass-transfer resistance [40]. The combination of different MIP-NPs resulted in the selective extraction of both analytes from complex food samples.

12.3.1.2 Chemical modification to immobilize nanoparticles in electrospun fibers He et al. prepared polyaniline coated silica nanofibers (PANI/SiO2) by combining electrospinning with in situ polymerization for the extraction of fluoroquinolones from honey [41]. After electrospinning of the silica nanofibers, aniline monomers were polymerized on the surface of the electrospun fibers. The extraction procedure was carried out by packing PANI/SiO2 nanofibers in a syringe. When combining silica nanofibers with PANI nanoparticles, the aggregation problems of PANI particles are eliminated, thus obtaining a uniform coating of PANI nanoparticles on the surface of the silica fibers. Morillo et al. developed PAN nanofibers to which superparamagnetic iron oxide nanoparticles (SPION) were fixed after hydrolysis of the electrospun fibers [42]. Nonhydrolyzed PAN fibers resulted in a low fixation of nanoparticles compared with the hydrolyzed fibers. The PAN-modified fibers were used for the removal of As(V) from wastewater. Liu et al. prepared a composite membrane from poly(vinyl alcohol)/poly(acrylic acid)/carboxyl-functionalized graphene oxide (GO) modified with silver nanoparticles (PVA/PAA/GO-COOH@AgNPs) for the catalytic degradation of the dye methylene blue [43]. Electrospun fibers containing graphene oxide were impregnated with silver nanoparticles by treatment with a silver nitrate

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solution containing ascorbic acid. The PVA/PAA/GO-COOH@AgNPs nanocomposite showed a significantly enhanced performance for the elimination of model dyes.

12.3.1.2.1

Nanoparticles in paper-based coated sorptive phases

The use of paper, a cellulose-based support, to fabricate thin film microextraction units is a low-cost and reliable alternative. Due to its porosity, it is typically employed for filtration purposes. The raw paper is, in general, incompatible with its immersion in liquid phases. However, if it is covered by a polymeric layer, a flexible and resistant device is obtained. As a general procedure, a piece of paper is immersed in a solution of the extraction phase (polymer), and after a number of dipping cycles, the polymeric layer is generated after solvent evaporation. Rios-Gomez et al. demonstrated that changing the dipping direction ensures a homogeneous distribution of the polymer (in this case polystyrene) which resulted in improved reproducibility for microextraction [44]. In this case, the polystyrene film was placed in a 100 mL pipette tip for the extraction of methadone from water. Zinc oxide nanorods have been synthesized onto paper by repeated immersion into a precursor solution. The functionalized extractant phase was used for the isolation of phenylurea herbicides from water [45]. The mechanical stability of the coating is the main challenge to overcome when NPs are used in the fabrication of paper-based sorptive phases. The nanomaterial must remain in the coating during the whole extraction process to obtain reproducible extractions. The inclusion of NPs in a polymeric network can be obtained by a simple solvent exchange [46,47]. Ríos-G omez et al. synthesized a paper-based phase with photocatalytic activity by this approach [48]. A segment of paper is immersed in a dispersion consisting of a polyamide dissolved in formic acid to which TiO2 NPs are subsequently added. Evaporation of the solvent induces the precipitation of the polyamide embedding the NPs in the polymeric network. The polymer entraps the NPs increasing their mechanical stability and enhances the sorption capacity of the paper-based membrane toward the target contaminants which are subsequently degraded by the photocatalytic action of the TiO2 nanoparticles. The use of a protective polymeric network is an efficient strategy to synthesize NPsbased coatings. However, Ríos-G omez et al. also investigated the synthesis of coatings consisting only of NPs [49]. The capacity of SWNHs to form stable and ordered aggregates (dahlia) paved the way to this development. For this purpose, the paper is immersed in a dispersion of SWNHs in chloroform. When the solvent evaporates, a mechanically stable coating of SWNHs is obtained.

12.4

Conclusions

Membranes have been used extensively in the analytical chemistry for the extraction and preconcentration of target compounds from different samples. Membrane modification by loading (nano)particles on commercial membranes (both hollow-fiber or planar formats), paper, or electrospun supports, effectively expands their range of applications. In fact, the use of planar configurations increases the surface available for interaction with the analytes which clearly enhances the efficiency of the (micro)

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extraction step. Nanoparticles can be used for the extraction as well as for environmental remediation if nanoparticles with catalytic activity are embedded in the polymeric network. Electrospun membranes are easily doped with NPs and the appearance of new fabrication approaches (like paper-based sorptive phases) will increase interest and drive further applications of particle-loaded membranes in coming years.

Acknowledgments Financial support for the Spanish Ministry of Economy and Competitiveness (MINECO) is gratefully acknowledged (Grant number CTQ2017-83175R).

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Fabric phase sorptive extraction: a new genration, green sample preparation approach

13

Abuzar Kabir, Kenneth G. Furton International Forensic Research Institute, Department of Chemistry and Biochemistry, Florida International University, Miami, FL, United States

13.1

Introduction

Although sample preparation has gained appreciable interests among separation scientists in recent years, the developments in analytical instruments has generally occurred at a faster pace than the progress in sample preparation [1]. The notable advances in modern analytical instrument are yet to be fully realized due to the absence of a universal sample preparation technique that can efficiently prepare the sample for instrumental analysis regardless of its origin, complexity, and the final chromatographic technique used for the separation, identification, and quantification of the target compounds. The importance of a universal and green sample preparation technique cannot be overstated. In fact, the success of an analytical method, developed for the analysis of target analytes at their trace and ultratrace level concentration present in complex sample matrices including environmental, pharmaceutical, food, and biological samples largely depends on the efficiency of the sample preparation technique used. The primary objectives of a sample preparation technique include: (a) selectively isolate and preconcentrate the target analytes from the complex sample matrix; (b) eliminate/minimize the matrix interferents from the sample; and (c) exchange the solvent so that the sample is compatible with the downstream chromatographic technique [2]. A clean, matrix interferentefree sample not only ensures easy separation, identification, and quantification of the target compounds but also safeguards the performance of the chromatographic system. Among many classical sample preparation techniques, liquid-liquid extraction (LLE) and solid phase extraction (SPE) are the most prevalent techniques [3e6]. However, a lack of selectivity, inability to discriminate the target compounds from matrix interferents, formation of hard to break emulsions, and use of large volumes of toxic and hazardous organic solvents are the prime concern for LLE. For many contemporary applications solid-phase extraction (SPE) is considered the gold standard sample preparation technique. SPE, however, is a laborious, time-consuming, and multistep sample preparation technique that demands clean, particle-free samples and often requires solvent evaporation and sample reconstitution in a suitable solvent, resulting in potential analyte loss. The limited number of polar sorbents for SPE is also a major

Solid-Phase Extraction. https://doi.org/10.1016/B978-0-12-816906-3.00013-3 Copyright © 2020 Elsevier Inc. All rights reserved.

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problem that hinders the selective extraction of numerous polar analytes and metabolites, especially from biological samples. In addition, protein precipitation is inevitable for biological samples prior to SPE that imposes an additional burden on the method. SPE is an exhaustive extraction technique that requires passage of the sample solution through a compact bed of sorbent particles dispensed either in the form of a cartridge or a disk. Due to the compactness of the sorbent particles, either positive or negative pressure must be applied to facilitate the steady flow through the sorbent bed and limits its field deployment for sampling and sample preparation. During the forced passage of the sample solution through the sorbent bed, unwanted matrix interferents also interact with the sorbent and become extracted. To eliminate the matrix interferents, a washing step is often employed after the extraction that may incur some analyte loss as well. SPE sorbents are primarily silica-based chemically modified sorbents with some polymeric phases such as hydrophilic-lipophilic balanced phase (HLB) also available [7]. To mitigate the shortcomings of SPE, solid phase microextraction (SPME) was introduced by Pawliszyn and coworkers in 1987 [8] as a solvent-free/solventminimized sampling and sample preparation technique. Numerous advantages of SPME compared to both SPE and LLE have promoted this technique toward high popularity in a short time. Unlike SPE, solid phase microextraction is an equilibrium driven sampling technique that utilizes predominantly organic polymer and particles blended with organic polymer as the sorbent, immobilized on the SPME fibers (fused silica fiber/stableflex fiber/metal fiber) by a free radical cross-linking reaction. Due to the lack of chemical bonding between the sorbent and the SPME fiber, the extraction phase is not generally stable to direct exposure to organic solvents. Most common SPME sorbents include polydimethylsiloxane (PDMS), polyacrylate (PA), polyethylene glycol (PEG), carboxen-polydimethyl siloxane (CAR-PDMS), divinylbenzene-polydimethylsiloxane (DVB-PDMS), and carbopack Z/PDMS [9]. For extracting the target analytes, SPME fiber coated with the polymeric sorbent is immersed into the headspace of the aqueous sample (HS-SPME) or directly immersed into the aqueous sample (DI-SPME). The analytes partition between the SPME sorbent and the aqueous solution based on the partition coefficient and the process continues until the equilibrium is reached. SPME is a selective extraction process and therefore does not require any washing step after the extraction. However, due to the viscous nature of the polymeric coating, the sample in SPME should be free of particles, debris, and proteins to avoid irreversible adsorption to the sorbent surface, leading to fouling of the sorbent coating. In addition, high viscosity of the polymeric sorbent in SPME often retards the analyte mass transfer into the sorbents and consequently prolongs the extraction time. SPME also suffers from poor extraction sensitivity due to the low phase volume (0.030e0.612 mL) [10]. Subsequently, a number of new microextraction techniques emerged with higher sorbent loading such as intube SPME [11], stir bar sorptive extraction (SBSE) [12], microextraction by packed sorbent (MEPS) [13], rotating-disk sorptive extraction (RDSE) [14], and thin film microextraction (TFME) [15]. Sorbent-based microextraction techniques are governed by two principle criteria: (a) thermodynamics and (b) kinetics [16]. The thermodynamic properties of the sorbent determine the maximum amount of analytes that can be extracted by unit

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mass of sorbent under a given set of extraction conditions. Since higher sorbent loading allows accumulation of a higher mass of analyte under equilibrium extraction conditions, sorbent loading should be maximized. However, the substrate type and geometry determine the maximum sorbent loading capacity for a microextraction 2device [17]. On the other hand, kinetics controls the rate of extraction and therefore the time needed to reach the extraction equilibrium. The faster the extraction equilibrium, the shorter is the sample preparation time and the higher is the throughput of the analytical laboratory. It is obvious that the extraction kinetics is directly related to the primary contact surface area (PCSA) of the device, which is defined as the surface area of the extraction device available for direct interaction with the analytes during the extraction process. The augmentation of PCSA also offers higher sorbent loading without changing the coating thickness and allows accumulating more analytes without saturating the sorption membrane. The sorbent coating technology is the key to improving the thermodynamic properties of sorbent materials. Classical organic/inorganic polymers such as poly(dimethylsiloxane) (PDMS), poly(ethylene glycol) (PEG) are highly viscous in nature. Mass transfer in these polymeric coatings is frustratingly slow, leading to prolonged extraction times. As such, increasing sorbent loading in microextraction device without changing the material properties does not significantly change the method sensitivity. As a result, stir bar sorptive extraction (SBSE) has failed to improve sensitivity significantly even though it has w250 times higher PDMS sorbent loading than SPME fibers [18]. A closer look on SPE and SPME sorbent/devices reveals that both the sample preparation techniques use an inert substrate (silica particles for SPE and fused silica/metal/ composite fiber for SPME) and an organic ligand (for SPE) or an inorganic/organic polymer/blended polymer (for SPME) as the sorbent. SPE primarily extracts the target analytes via a flow through system. On the other hand, SPME primarily extracts analytes from either headspace or by direct immersion. Elution of the analytes in SPE is carried out using a suitable organic solvent while for SPME fibers, thermal desorption is more common. Although, the primary objectives of both SPE and SPME are the same, their approach in achieving these objectives is profoundly different in (a) extraction mechanism; (b) sorbent material chemistry; and (c) analyte elution/ desorption process. As such, the goal for the development of a new sample preparation technique should be devoted to the unification of both SPE and SPME so that the best of both the techniques can be beneficially exploited. Some other shortcomings of SPE and SPME include (a) inherently low thermal, solvent, and chemical stability of the sorbents; (b) limited selectivity of the sorbents; (c) slow mass transfer of analytes due to the high viscosity of the polymeric sorbents; (d) absence of cation-exchanger, anion-exchanger, and mixed mode sorbents for SPME that limits the SPME applications to only neutral analytes unless matrix pH adjustment is used. Taking all the shortcomings of SPE and SPME into consideration, fabric phase sorptive extraction (FPSE) has been developed as a universal sample preparation technique that innovatively combines the extraction mechanism of SPE and SPME into a single sample preparation platform [19e21]. FPSE has eloquently addressed most of the shortcomings of SPE and SPME and combines almost all the features of both

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techniques. FPSE utilizes a section of fabric (cellulose, polyester, or fiberglass) as the substrate with a sol-gel sorbent coating on its surface. The sorbent coatings are chemically bonded to the substrate for high pH and sorbent stability. When sol-gel sorbents are coated on fiberglass, they offer high thermal stability that allows thermal desorption of the extracted analytes via instantaneous heat shock (Thermal Desorption Unit). To extract the target analytes, FPSE membranes can be inserted directly into the sample matrix such as whole blood, urine, whole milk, environmental water without any sample pretreatment. The extraction rate is often enhanced by convection using a Teflon coated magnetic stir bar. After the extraction, the analytes are eluted in a small volume of organic solvent in a small back-extraction tube. Sponge-like porous architecture of the sol-gel sorbents allows fast diffusion of the aqueous sample for rapid analyte extraction as well as the organic solvent for quick elution of the analytes. Selection of a suitable elution solvent allows injecting an aliquot of the sample into gas chromatography (GC), liquid chromatography (LC), capillary electrophoresis (CE), and inductively coupled plasma-mass spectrometry (ICP-MS) to maximize the analytical information. The current chapter describes the rationale of the invention of FPSE, building blocks of FPSE membranes, development and characterization of FPSE membranes, extraction principles exploited in FPSE, FPSE method development process, FPSE protocol, and different implementations of FPSE. Applications of FPSE are also briefly discussed.

13.2

Building blocks of fabric phase sorptive extraction membranes and their role in extraction

Fabric phase sorptive extraction membranes are built on the following building blocks: (1) a fabric substrate; (2) a sol-gel inorganic precursor/organically modified inorganic precursor; (3) a sol-gel active inorganic/inorganic polymer. Occasionally, high surface area carbonaceous particles such as carboxen, graphene, carbon nanotubes are also used in combination with inorganic/organic polymers to enhance the selectivity of the extracting sorbents. Fig. 13.1 presents a schematic representation of sol-gel sorbent-coated FPSE membrane. The description of different building blocks of FPSE membranes are presented below: (i) Fabric substrate:

Fabric substrate plays a key role in building the FPSE membrane. Unlike other extraction and microextraction techniques, the substrate in FPSE is not merely a host for the sorbent but also actively contributes to the overall selectivity of the FPSE membrane via hydrophilic/hydrophobic interactions. Among many potential candidates, cellulose, polyester, and fiberglass fabrics are primarily used as the substrates for FPSE membranes. All these fabrics possess sol-gel active functional groups on which the sol-gel sorbent networks are chemically bonded during the sol-gel

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Figure 13.1 Schematic representation of FPSE membrane and its different building blocks.

coating process. Since the sorbent loading on the FPSE membrane depends on the concentration of sol-gel active functional groups, the amount of sorbent loading per unit area on cellulose fabric is substantially higher than polyester fabric. It is important to note that the sol-gel sorbent networks chemically bind to the sol-gel active functional group, leaving the majority of the fabric substrate surface uncovered and available for interaction with target compounds. Due to the porosity of the fabric, FPSE membranes mimic a solid phase extraction disk. During the extraction process, aqueous sample permeates through the FPSE membrane and facilitates rapid and near exhaustive extraction. Fig. 13.2 presents the chemical structures of different FPSE substrates. (ii) Inorganic/organically modified sol-gel precursor:

Inorganic/organically modified sol-gel precursor plays an important role in the sol-gel sorbent coating process. It creates the 3D networks of sol-gel sorbent by randomly incorporating the inorganic/organic polymer into the networks. It also acts as a linker to bind the sol-gel sorbent networks with the fabric substrate. When organically modified inorganic sol-gel precursors are used, the organic pendant moiety

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(A)

(B)

(C)

Figure 13.2 Chemical structures of (A) a polyester unit; (B) a cellulose unit; and (C) fiberglass fabric.

actively contributes to the overall selectivity of the FPSE membrane. As such, the polarity and selectivity of the FPSE membrane can be fine tuned by a judicious selection of the sol-gel precursor. Among a large number of available sol-gel precursors, tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), methyl trimethoxysilane (MTMOS), phenyltrimethoxysilane (PTMOS), 3-aminopropyltrimethoxysilane (3APTMOS), octadecyltrimethoxysilane (C18-TMOS), octyltrimethoxysilane (C8TMOS), titanium isoproxide, zirconium isopropoxide, teramethoxygermane are noteworthy. (iii) Sol-gel active inorganic/organic polymer:

Sol-gel active inorganic polymers such as poly(dimethylsiloxane) (PDMS), poly(dimethyldiphenylsiloxane), or organic polymers such as poly(ethylene glycol), poly(tetrahydrofuran) are randomly integrated into the sol-gel networks via sol-gel synthesis and act as the primary source of the selectivity and extraction affinity toward the target analyte(s). The polymer used in the sol-gel sorbent coating process provides different intermolecular interactions, which are attributed to their structure and broadly differ from one polymer to the other.

13.3

Preparation of sol-gel sorbent coated fabric phase sorptive extraction membranes

Due to the strong covalent bonding between the sorbent and the fabric substrate, FPSE membranes demonstrate excellent thermal, solvent, and chemical stability. As such, sol-gel sorbent coated FPSE membranes can be exposed to any organic solvent, strong

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acidic or basic environment (pH 1-12), and high temperature (w350
C) for thermal desorption of the extracted analytes (only sol-gel sorbents coated on fiberglass). Incorporation of hundreds of organic polymers/ligands into a large number of inorganic polymeric networks opens the floodgates for thousands of novel sorbents with different selectivity, surface morphology, and other advanced material properties [22e25]. Preparation of sol-gel sorbent coated fabric phase sorptive extraction membrane involves a number of sequential steps including: (1) selection and pretreatment of fabric phase sorptive extraction membrane; (2) design and preparation of the sol solution for sol-gel sorbent coating on the substrate; (3) sol-gel sorbent coating process using immersion coating technology; (4) aging, conditioning, and cleaning of sol-gel sorbent coated fabric phase sorptive extraction membrane; and (5) cutting the FPSE membrane into an appropriate size.

13.3.1 Selection and pretreatment of fabric phase sorptive extraction membrane In order to qualify as a substrate for FPSE, the fabric should meet the following criteria: (a) presence of abundant surface hydroxyl groups; and (b) surface permeability so that aqueous solution can easily permeate through the FPSE membrane during analyte extraction and organic solvent during elution/back-extraction. Many commercially available fabrics meet these criteria and among them cellulose, polyester and fiberglass, nylon, polyamide fabric are noteworthy. The commercial fabric often retains residual finishing chemicals on their surface used to provide glossiness and protect the fiber from accumulating dust particles. These residual chemicals must be removed prior to the sol-gel sorbent coating on the fabric surface. In addition, the surface hydroxyl groups should be activated. To clean the fabric and activate surface hydroxyl groups, a fabric pretreatment regimen was developed. The detailed pretreatment process is described elsewhere [2,21].

13.3.2 Design and preparation of sol solution for sol-gel sorbent coating on the substrate The design of the sol solution primarily depends on the nature of the target analytes and the sample matrix. Primary focus is on the selection of sol-gel precursor(s) and the organic polymer. If the analytes are hydrogen bond donors or acceptors, the organic polymer should possess hydrogen bond acceptor or donor groups for complementary interactions with the analytes.

13.3.3 Sol-gel sorbent coating process using immersion coating technology Sol-gel sorbent coating process involves the following sequential reactions: (i) Catalytic hydrolysis of sol-gel precursor or organically modified sol-gel precursor;

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Solid-Phase Extraction

(ii) Polycondensation of hydrolyzed precursor, resulting in a growing sol-gel network; (iii) Random incorporation of sol-gel active polymer into the growing sol-gel network on the fabric substrate surface via polycondensation.

Sol-gel reactions are generally carried out under ambient conditions. Typically, methyltrimethoxysilane (MTMS) is used as the sol-gel precursor and trifluoroacetic acid (TFA) (95%, 5% H2O) as the sol-gel catalyst. During the hydrolysis, the three methoxy groups of MTMS are hydrolyzed to form hydroxyl groups and subsequently begin forming a 3D network via polycondensation. During the formation of 3D network, sol-gel active organic polymer randomly enters the network. Finally, the sol-gel sorbent network chemically bonds to the fabric substrate through the fabric hydroxyl groups via polycondensation. Fig. 13.3 illustrates the different chemical reactions involved in the sol-gel gel sorbent coating process. The sol-gel sorbents are highly porous and possess a sponge-like porous architecture. Due to the high porosity of sol-gel sorbent and the permeability of the fabric substrate, aqueous sample travels through the FPSE membrane thousands of times during the extraction process and ensures exhaustive extraction of the target compounds in a relatively short time. Sol-gel sorbent coating on fabric substrate is generally carried out at room temperature in a 2 oz reaction vessel for 6-8 h. Subsequently, the sol solution is drained from the reaction vessel, the sol-gel sorbent coated FPSE membrane is dried in air for 1h, and then transferred to a home-built thermal conditioning device.

(A)

(B)

(C)

(D)

Figure 13.3 Schematic representation of sol-gel poly(dimethyldiphenylsiloxane) coating on a polyester fabric.

Fabric phase sorptive extraction: a new genration, green sample preparation approach

363

13.3.4 Aging, conditioning, and cleaning of sol-gel sorbentcoated fabric phase sorptive extraction membrane The objective of aging and conditioning of the sol-gel sorbent-coated FPSE membrane is to ensure that the sol-gel reaction goes to the completion. The aging and conditioning is carried out inside a homemade conditioning unit at 50
C under continuous flow of helium gas for 24 h. The conditioning is followed by cleaning the sol-gel sorbentcoated FPSE membrane with methanol/methylene chloride (50:50 v/v). During the cleaning process, any unreacted precursor, polymer, reaction intermediates are removed. After cleaning, the FPSE membrane is conditioned again for 12 h at 50
C.

13.3.5 Cutting the FPSE membrane into appropriate size Unlike other sample preparation techniques, one of the major advantages of FPSE is its variable size that can be tailored to specific applications. For example, due to the small volume of typical biological samples such as whole blood and plasma, FPSE membrane disks of 1 cm2 are used. For environmental samples, an FPSE membrane of 2.5 cm2 is typically used. The sol-gel sorbent-coated FPSE membranes are cut into sizes using a homemade cutting device [26,27].

13.4

Sol-gel sorbents for fabric phase sorptive extraction

Among all extraction and microextraction techniques, FPSE has developed the highest number of sorbents, from nonpolar to medium polar to polar to cation exchanger to anion exchanger to zwitterionic to mixed mode chemistries. FPSE also is the first microextraction technique that offers anion exchanger, cation exchanger, zwitterionic and mixed mode sorbents. Table 13.1 provides a list of fabric phase sorptive extraction sorbent chemistries with other pertinent information.

13.5

Characterization of fabric phase sorptive extraction membranes

Sol-gel sorbent-coated fabric phase sorptive extraction membranes are often characterized by a number of techniques including scanning electron microscopy (SEM), atomic force microscopy (AFM), thermogravimetric analysis (TGA), Fourier transform-infrared spectroscopy (FT-IR), nitrogen adsorption isotherm, elemental analysis, energy dispersion spectroscopy (EDS). SEM provides details of the surface morphology of the fabric substrate, before and after the sol-gel sorbent coating. FT-IR spectra provide information on the chemical integration of different sol solution ingredients into the sol-gel sorbent network as well as integration of sol-gel sorbent network with the fabric substrate.

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Solid-Phase Extraction

Table 13.1 List of major fabric phase sorptive extraction sorbents.

FPSE sorbent

Sorbent type

Sorbent loading (mg/cm2)

1. Sol-gel polydimethylsiloxane

Nonpolar

2.30

2. Sol-gel polydimethyldiphenylsiloxane

Nonpolar

1.93

3. Sol-gel ppolytetrahydrofuran

Medium polar

3.96

4. Sol-gel polyethylene glycolpolypropylene glycol-polyethylene glycol

Medium polar

5.68

5. Sol-gel polypropylene glycolpolyethylene glycol-polypropylene glycol

Medium polar

5.25

6. Sol-gel methacrylate

Medium polar

4.51

7. Sol-gel C4

Medium polar

1.90

8. Sol-gel C8

Medium polar

2.33

9. Sol-gel C12

Nonpolar

3.54

10. Sol-gel C18

Nonpolar

4.88

11. Sol-gel polycaprolactone diol

Polar

3.46

12. Sol-gel polycaprolactone triol

Polar

N/A

13. Sol-gel polycaprolactonepolytetrahydrofuranpolycaprolactone

Medium polar

4.64

14. Sol-gel polycaprolactonepolydimethylsiloxanepolycaprolactone

Medium polar

6.14

15. Sol-gel UCON

Polar

N/A

16. Sol-gel graphene

Nonpolar

N/A

17. Sol-gel multiwalled carbon nanotubes

Nonpolar

N/A

18. Sol-gel sucrose

Polar

N/A

19. Sol-gel sucralose

Polar

N/A

20. Sol-gel chitosan

Polar

N/A

21. Sol-gel carbowax 20M

Polar

8.64

22. Sol-gel polyethylene glycol 10,000

Polar

6.36

23. Sol-gel polyethylene glycol 300

Polar

4.45

Fabric phase sorptive extraction: a new genration, green sample preparation approach

365

Table 13.1 List of major fabric phase sorptive extraction sorbents.dcont’d

FPSE sorbent

Sorbent type

Sorbent loading (mg/cm2)

24. Sol-gel cation exchanger

Cation exchanger

N/A

25. Sol-gel anion exchanger

Anion exchanger

N/A

26. Sol-gel zwitterionic sorbent

Cation and anion exchanger

N/A

27. Sol-gel mixed mode sorbent

Medium polar, cation exchanger; medium polar, anion exchanger

N/A

28. Sol-gel zwitterionic-mixed mode sorbent

Medium polar, cation exchanger, anion exchanger

N/A

13.6

Working principle of fabric phase sorptive extraction

Solid sorbent-based sample preparation techniques are primarily classified into two major classes based on their extraction mechanism: (a) exhaustive extraction, as in solid phase extraction (SPE); and (b) equilibrium extraction, as in solid phase microextraction (SPME). Exhaustive extraction techniques employ extraction of the analytes during percolation of the sample through the sorbent bed. However, during this process many unwanted compounds from the sample matrix may also become adsorbed. Removal of these materials is necessary prior to chromatographic analysis and is often accomplished by incorporating a washing step into the workflow prior to eluting the analytes. The washing step may result in substantial analyte loss and should be avoided if possible. Equilibrium extraction techniques, such as SPME, require positioning the device inside the sample matrix (direct immersion extraction) or in the headspace of the sample (headspace extraction) and the mass transfer of analytes continues until the equilibrium is reached. Due to the viscous nature of the extracting phases, a prolonged extraction time is required to achieve equilibrium. In addition, low sorbent mass (especially in SPME) often fails to accumulate a high mass of target analytes during the extraction, resulting in a lower overall method sensitivity. Fabric phase sorptive extraction, on the other hand, integrates both extraction mechanisms. When the FPSE membrane is immersed into the sample during extraction, it mimics direct immersion solid phase microextraction. The coated fabric substrate is inherently permeable. In a stirred solution, the sample rapidly permeates through the porous bed of the FPSE membrane mimicking an SPE disk. This permeation occurs thousands of times during the extraction period. Analytes are extracted by the FPSE membrane almost exhaustively without matrix interferents due to the unique selectivity of the sorbent.

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13.7

Solid-Phase Extraction

Fabric phase sorptive extraction protocol

Fabric phase sorptive extraction has profoundly simplified the overall sample preparation workflow. It has successfully eliminated all the sample pretreatment steps such as filtration, centrifugation, protein precipitation (for biological samples), and defatting (milk samples). It also eliminates post sample treatment such as solvent evaporation and sample reconstitution. The simplification of the sample preparation process not only saves time, effort, money, and organic solvent usage, but also improves the overall quality of the analytical data. Fig. 13.4 illustrates the different steps involved in fabric phase sorptive extraction. The fabric phase sorptive extraction protocol is described below: Step 1.

Cleaning of FPSE Membrane

(i) Prior to the extraction, immerse all the FPSE membranes in 2 mL of acetonitrile:methanol (50:50 v/v) for 5 min (no shaking/vortexing/stirring is needed). Membranes are manipulated using clean tweezers. (ii) Rinse the FPSE membranes in 2 mL DI water to remove residual organic solvents. (iii) Now the FPSE membranes are ready for extraction. Step 2.

Extraction (10-40 min):

(i) Transfer a representative aliquot of the sample to a 10/20 mL SPME vial. (ii) Insert a clean Teflon-coated magnetic stir bar into the vial. (iii) Insert a clean FPSE membrane into the vial.

1

2

3

4

5

6

7

8

Figure 13.4 Steps in fabric phase sorptive extraction (FPSE): (1) unfiltered, dirty environmental sample; (2) FPSE media on watch glass; (3) an FPSE medium on a tweezer; (4) fabric phase sorptive extraction; (5) back extraction in organic solvent; (6) transferring preconcentrated eluent into sample vial; (7) analysis in GC; (8) analysis in HPLC [2].

Fabric phase sorptive extraction: a new genration, green sample preparation approach

367

(iv) Set up magnetic stirrer to medium agitation to ensure adequate analyte convection. (v) Extract for 10-40 min depending on the type of target analyte(s). (vi) Remove the FPSE membrane from the sample and dry it by gently pressing against a paper towel. Step 3.

Elution/Back extraction (5-10 min):

Use a deactivated glass vial/Eppendorf tube for analyte desorption. Fold the FPSE membrane and insert it into the desorption vial. Add 300-500 mL of solvent into the vial. Allow 5-10 min for complete back extraction (unnecessity to vortex, centrifuge, or sonicate). (v) Centrifuge the back-extraction solution if it looks cloudy or contains particulates. (vi) Pipette the back-extraction solvent/eluent into a GC/LC sample vial for chromatographic analysis.

(i) (ii) (iii) (iv)

Step 4.

Cleaning FPSE membrane and storing for future use:

(i) After the elution/back extraction, immerse the FPSE membrane in 2 mL acetonitrile:methanol (50:50 v/v) for 5 min (no shaking/vortexing/stirring is necessary). (ii) Dry the FPSE membrane to drive off the solvents by putting it on a watch glass for 510 min. (iii) Store in an airtight glass container for future use.

13.8

Fabric phase sorptive extraction method development

A number of factors that affect the FPSE selectivity and extraction efficiency should be optimized. These factors include: (a) FPSE sorbent chemistry; (b) sample volume; (c) stirring speed; (d) extraction time; (e) addition of salt; (f) elution solvent type, volume, and elution time; and (g) extraction temperature. The selection of an appropriate sorbent is generally the most important step that determines the overall sensitivity of the method. In order to simplify the sorbent selection process, a second order mathematical model has been developed for each of the FPSE sorbent that allows calculating absolute recovery (%) of an analyte using its log Kow value. These models are valid for log Kow values between 0.3 and 5.07. Based on the log Kow values of the analytes, three to four potential sorbent candidates can usually be identified for systematic method development experiments. Fig. 13.5 illustrates this process for a sol-gel polycaprolactone-polydimethylsiloxane-polycaprolactone coated FPSE membrane and the corresponding absolute recovery versus log Kow correlation model. Table 13.2 presents the absolute recovery calculation equations for representative FPSE sorbents.

368 Solid-Phase Extraction

Figure 13.5 Schematic representation of sol-gel silica polycaprolactone-polydimethylsiloxane-polycaprolactone coated FPSE membrane (left) and the associated absolute recovery versus log Kow correlation model (right).

Fabric phase sorptive extraction: a new genration, green sample preparation approach

369

Table 13.2 Absolute recovery calculation equations for representative FPSE sorbents. Sorbent (substrate)

Absolute recovery equation

Si-CW20M (cellulose)

R, % ¼ 4.298 þ 22.823*log Kow  3.134*(log Kow  2.737)2

Si-PEG1000 (cellulose)

R, % ¼ 11.535 þ 20.950*log Kow  0.402*(log Kow  2.737)2

Si-PEG300 (cellulose)

R, % ¼ þ14.759 þ 16.310*log Kow  5.550*(Log Kow  2.737)2

Si-CN-CW20M (cellulose)

R, % ¼ 24.393 þ 23.940*log Kow þ 1.247*(log Kow  2.737)2

Si-PPG-PEG-PPG (cellulose)

R, % ¼ 3.649 þ 21.546*log Kow 2.879*(Log Kow  2.737)2

Si-PEG-PPG-PEG (cellulose)

R, % ¼ 7.680 þ 23.069*log Kow 1.726*(Log Kow  2.737)2

Si-PTHF (cellulose)

R, % ¼ þ12.401 þ 17.849*log Kow þ 17.849*(Log Kow  2.737)2

Si-PTHF (fiber glass)

R, % ¼ 28.442 þ 20.950*log Kow þ 3.327*(Log Kow  2.737)2

SieC18 (cellulose)

R, % ¼ 2.275 þ 20.816*log Kow  4.148*(Log Kow  2.737)2

SieC8 (cellulose)

R, % ¼ 3.393 þ 21.261*log Kow  3.772*(log Kow  2.737)2

Si-PDPS (cellulose)

R, % ¼ 10.300 þ 17.450*log Kow  0.288*(log Kow  2.737)2

Si-PDMDPS (polyester)

R, % ¼ 9.185 þ 17.816*log Kow  1.966*(log Kow  2.737)2

Si-PDMDPS (cellulose)

R, % ¼ 19.602 þ 15.454*log Kow  1.622*(log Kow  2.737)2

Once the most suitable sorbent chemistry is identified, a systematic method development approach can be followed to optimize the other extraction parameters. A detailed approach is presented elsewhere [2,28e30].

13.9

Advantages of fabric phase sorptive extraction over conventional sorbent-based sample preparation techniques

Fabric phase sorptive extraction has substantially simplified sample preparation process by eliminating all sample pretreatment and post-treatment steps typically involved in conventional sample preparation techniques. A schematic comparison between SPE, SBSE, and FPSE illustrates the operational advantages of FPSE over other major sample preparation techniques (Fig. 13.6 and Table 13.3) [28].

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SOLID PHASE EXTRACTION

FABRIC PHASE SORPTIVE EXTRACTION

STIR BAR SORPTIVE EXTRACTION

5.0 mL milk

9.9 mL milk

1 g milk ACN solution of surrogates

Add 17.5 mL NaCl in extracon vial

Added EDC standard solution Cleaning FPSE media in MeOH

2.0 mL 0.2M acetate buffer

Condioning the SPE cartridge

Vortex, 1 min

Add wash soluon

Shake and add 2.5 mL MeOH

Drying SPE cartridge

Add 7.5 mL ACN 7.5 mL PP soluon

Add DI water to make 50 mL total volume

Vortex, 1 min

Sample loading

Vortex, 1 min

Add Twister sr bar and extract 24 h

FPSE for 50 min

Centrifuge, 4000 rpm, 5 min

Washing the cartridge

Centrifuge, 5000 rpm, 10 min

Remove Twister, wash with water

Washing FPSE media in DI water and drying with a ssue

Separate supernatant

Eluon using 2 mL MeOH

Separate liquid layer and filter

Elute in ACN with sonicaon, 5 min

Add DI water to make total volume 25 mL

Solvent Evaporaon

Evaporate to dryness

Evaporate to dryness

Sample reconstuon

Re-dissolve in 20 mL DI water

Re-dissolve in 50 μL mobile phase

Heat at 60°C, 20 min

Analysis by HPLCDAD

Analysis by GC-MS

Eluon in ACN 50 μL ethyl acetate BSTFA/TMCS

Filtraon

Analysis by LC-UV

Figure 13.6 Comparison between solid phase extraction, stir bar sorptive extraction, and fabric phase sorptive extraction [28].

Analysis by LC-MS/ MS

Solid-Phase Extraction

Analysis by HPLCDAD

Rinsing FPSE media in DI water

SPME

SPE

1. Substrate geometry

FPSE utilizes a small piece of cotton fabric as the substrate that holds the sol-gel-derived hybrid organic-inorganic polymeric sorbent responsible for analyte extraction from different matrices.

A small segment of 100 mm i.d. fused silica rod mounted on the tip of a stainless steel tube holds the polymeric sorbent material. Easy to use, but prone to needle bending and fiber breakage if the operator is not properly trained.

Small, spherical silica particles are used to support the coating of sorbent materials on their surfaces. The coated silica particles are then packed in different geometrical formats including cartridges, disks, etc. The sorbent bed often exerts enough resistance to flow aqueous solution containing the target analyte(s) requiring application of either positive or negative pressure in the extraction process.

2. Accessible surface area

Provides flexible, open, flat, and easily accessible surface for sorbent-analyte interaction. Typically, a 5 cm2 piece of fabric is used as the extraction phase. Two sides of the fabric offer approximately 10 cm2 directly accessible sorbent coated surface.

Due to the small surface area of the substrate, the sorbent-analyte contact surface area is approximately